Tantantara! Tzing, boom!

Is this any way to celebrate a centennial?

Here it is the hundredth year of that undying pronouncement: ''It so happens that if there is an institution in Great Britain which is not susceptible of any improvement at all, it is the House of Lords.''

But in February came the final curtain for the financially troubled D'Oyly Carte Opera Company, in whose 1882 premiere of Gilbert and Sullivan's ''Iolanthe'' these words were originally spoken.

Now comes a new act in the Labour Party's long-running melodrama of trying to improve the House of Lords out of existence.

Reluctantly recognizing that abolition of the Lords might not get first priority amid Britain's other present challenges, Labourites reportedly are considering a compromise. For example, the next Labour government, if there is one, might simply recommend to the Queen the creation of enough new peers to ensure a majority in the Lords.

Not even left-wing abolitionist Tony Benn offers the simple solution in ''Iolanthe'': a bill to open the peerage to competitive examination. Perhaps he anticipates the snag noted in the operetta: ''With a House of Peers composed exclusively of people of intellect, what's to become of the House of Commons?''

But Conservatives as well as Labourites over the years have been trying to do something to make the Lords more democratic. It's not that anyone would deny Gilbert and Sullivan's ode to the ''nobly born'' and ''well-connected''--''High rank invokes no shame,/ We boast an equal claim/ With him of humble name/ To be respected.'' It's just that to bestow legislative power through birth or patronage has been rubbing against the grain in the Britain of today.

A few years ago a group of Conservatives suggested the end of hereditary rights to membership in the Lords. It proposed that two-thirds be elected and the rest nominated through the crown.

Not that the Lords has all that much power. It has none over money bills. It can affect others mainly through delay.

But the House of Lords has been judged by many to serve a role as guardian of Britain's unwritten constitution and traditions, resisting what it sees as parliamentary encroachments on them. Thus, for all the satire to which the woolsack and panoply have been subjected, Britons tend toward reform rather than elimination of the Lords.

Another hundred years will probably go by with these ''paragons of legislation, pillars of the British nation'' still going strong in some fashion. Tantantara! Tzing, boom!