Great Britain; British 'Occasions' - royal, sporting, and civic - exemplify a way of life

It's ''set in a silver sea,'' according to one of its poets, this group of islands somewhere north of France and southwest of Norway. The inhabitants, by and large, are somewhat shy and private.

They speak a language related to ''American'' - with a staggering number of local variations, though in the ''higher'' echelons of their society there is a long-established attempt to standardize usage. They call this ''Queen's English, '' and at its most refined it involves a highly skilled kind of intonation achieved (according to an old Etonian I saw on TV once) without perceptibly opening the mouth. The resultant sound is not only incalculably superior, but also wonderfully difficult to understand.

Of course, tourists often encounter language difficulties. But these force them to the intriguing alternative of looking rather than listening. Fortunately , in this group of islands they can see quite a lot. As is often the case with insular, reticent peoples, there is here a longstanding tradition of staging - often in the most ceremonial, theatrical ways - what might be summarized as ''Occasions.'' These islands display all kinds of Occasions - royal, sporting, artistic, shooting, civic, ceremonial, and horticultural. Anything that can be turned into an Occasion (the more unusual and exclusive the better) is turned into an Occasion in Britain.

How seriously do we Britons take our Occasions - our Royal Openings of Parliament, our Trooping the Colour, our Henley Royal Regattas, our Buckingham Palace garden parties?

Is it possible for ordinary Britons to sit on a park bench on a summer's day and let all these ancient and important events take place without giving them a thought?

The answer is bound to be paradoxical. After all, none of these occasions could occur against the will of the ordinary people - and go on occurring into the 1980s. Clearly we must love our customs. They are definitely more to us than just good tourism - though that too. The answer must be that we take them very seriously indeed - and not very seriously at all.

To try to understand such a paradox could be a revealing exercise for a tourist to Britain.

How is it that this country still fosters a hierarchy of people important by birth from the Queen downward? What is it in the British character that enables its ''upper class'' to go on existing? Why have we still got what even Prince Charles has described as ''something as curious as the monarchy''? What is it that causes such a vast number of us to make a tall and fashionable woman named Diana into a national symbol, so much so that she's never out of the papers?

It has to be because we Britons have what may be a unique capacity for combining our sense of humor with our sense of honor.

We find the whole notion of an upper class both irresistibly solemn and irresistibly funny. To visit these shores without experiencing at least one upper-class Occasion would be perhaps to miss the very essence of the country.

The book to equip yourself with is ''Debrett's Guide to Britain: Where to Go and What to See'' (edited by Joanna Household: published by Putnam at $24.95.)

''The book,'' Mrs. Household told me, ''doesn't claim to be comprehensive. The idea is to point people in the right direction.'' Nevertheless it does present in some detail what used to be called (and apparently, in certain circles, still is called) ''the events in the social calendar.'' Arranged by month, the upper-crust occasions it describes range from the Oxford and Cambridge Boat Race to the Chelsea Flower Show, from the Garter Day Ceremony at Windsor Castle to the Eton and Harrow Cricket Match, from Oxford May Morning to Swan Upping on the Thames, from Highland Games to the Last Night of the Proms.

In the introduction there is talk of ''rubbing shoulders with the royal family'' at an event such as the Badminton Horse Trials. I asked Mrs. Household if the phrase ''rubbing shoulders'' wasn't an exaggeration. ''Well, the amazing thing is,'' she replied, ''they really do wander about and mix with people at Badminton.'' Members of the royal family also always show up at the Highland Gathering each September at Braemar, near Balmoral, Scotland. And they appear in May at the big London balls, particularly the Rose Ball and the Caledonian Ball at the Grosvenor House Hotel on Park Lane.

Treating its subject with, on the whole, proper seriousness, the Debrett's guide offers a surprising amount of purely practical advice. At the Caledonian Ball, for instance, it emphasizes ''strictly formal dress - white tie for men and long ball gowns for women, with Highland dress for everyone entitled to wear tartan. . . . Debutantes will usually wear their traditional long white evening dresses. Tiaras are not usually worn. . . .'' Are standards, one wonders, falling?

At the Boat Race (and this annual race on the Thames between crews from Oxford and Cambridge, which the latter has now lost for a shameful nine years in succession, is known as THE Boat Race, presumably because it is the only one that counts) advisable footwear for bystanders is ''boots or shoes that will stand up to slush, crowd trampling, and cold.''

And there are other useful tips. At the aforementioned Badminton Horse Trials , the ''right place'' to be on Saturday (the ''great day'') is the West Stand. Visitors are warned not to bring dogs, small children, or grandparents - unless the latter are of the ''type peculiar to the British upper classes, which never tire.''

Helpfully for the uninitiated, one is prompted to ''leap to the side of the course at the sound of a whistle, for it means a horse is coming.''

It would seem, on the whole, that any tourist wanting to attend an Occasion which might also include royalty would be well to develop an interest in the horse. A great many events revolve round this noble creature, so affectionately regarded by a majority of royals. From the Royal Windsor Horse Show to Royal Ascot (no, it couldn't actually look like ''My Fair Lady'' could it?), from the British Jumping Derby at Hickstead to the Horse of the Year Show, there is scarcely a month without one or more major horsy occasions. Often enough this means the presence of the Queen, the Queen Mother or Princess Anne. There's polo , of course, with the Prince of Wales as a star participant.

Perhaps what amazes people is the continuance of occasions which not only seem to belong to a different era, but to a different world.

The book lets one into some of the mysteries and obscure origins and meanings of things in this world: a world where the ''May balls'' at Cambridge colleges take place in June; where the ''Lord's Test'' turns out to be a top cricket match; where ''Agar's Plough'' is the name of the playing fields of an exclusive school; where the Queen's official birthday is celebrated on a different date from her real one because the ceremony to celebrate it (the Trooping of the Colour) is held in summer since there is ''then at least a chance of reasonable weather.''

There is, however, one unsolved mystery. That is the Eton Wall Game, a muddy and ''remarkable spectacle'' which is, we are told, ''incomprehensible.''

I asked Mrs. Household if it really is possible to make one's way to some of the more exclusive occasions listed in her book. ''Well,'' she said quotably, ''I don't really like to be quoted, but it is amazing how easily a really determined person can get into some of these things . . .'' Some do present bigger problems than others, however. At the royal garden parties, for example, you must have an invitation.

For the State Opening of Parliament you can only attend as a guest of a peer. But many occasions can be easily seen by lining roads like the Mall (royal events are forecast on the back page of the Times), or by sitting on hillocks eating smoked salmon out of a well-stocked picnic hamper, or by standing in Wellington boots beside rivers. Some occasions can be discreetly gate-crashed so long as a few stares don't embarrass you. And a very large number of them can be experienced by buying tickets or joining an appropriate society. But a warning. The almost universal rule is this: plan months ahead. (The Braemar gathering, on Sept. 1 this year, has to be booked before mid-May.)

The addresses and phone numbers in ''Debrett's Guide to Britain'' are accurate and useful. Mrs. Household added an encouraging verbal footnote: ''I learnt a lot,'' she told me, ''editing this book - particularly how seriously sporting people take their interests, but in the nicest way. They are always happy that others are genuinely interested.'' In these areas one may well still encounter devoted amateurs in the original meaning of the word. They do these things because they love 'em. And you will (almost always) be welcome. In the most exclusive sort of way, of course (even without a tiara).