London's bumper year for tourists is a bumper year for swindlers

It looks like a bumper year for London's growing force of swindlers, who are making a handsome living from cheating overseas visitors to the capital. With the tourist season at its height, illegal street traders, unlicensed cab drivers, and phony photographers are swindling unsus-pecting tourists out of millions of pounds. These unscrupulous operators are cashing in on the upswing in the number of overseas visitors after several lean years for the British tourist trade.

Americans in particular are flocking back and spending freely. Last year they brought $375 million worth of trade to London. They are among the prime targets of the swindlers who work in the main shopping districts of the West End and around such top tourist attractions as Trafalgar Square, Westminster Abbey, and the Kensington museums.

As a local official in London said: ''The situation in the West End is fairly critical. The more tourists we get, the more crooks we have on the streets. They take advantage of Londoners' honest reputation, palm tourists off with substandard or counterfeit goods, and end up by giving the city a bad name.''

For many, the rip-off begins as soon as they set foot in Britain. Unlicensed cab drivers, in sedans rather than the familiar London black taxi, pounce on tourists at Heathrow Airport and at major railway terminals. Victoria Station, with its direct rail link from Gatwick Airport, is a favorite haunt.

Despite special squads of plainclothes policemen, these drivers often intercept travelers on their way to the ranks of regular black taxis. They work without meters and charge phenomenal rates.

Scotland Yard's cab law enforcement office says it has been deluged with complaints. It cites the recent case of an elderly American who paid (STR)200 (some $300) for a trip from Heathrow to a central London hotel - well over 10 times the normal fare.

Droves of illegal street traders, working the main shopping streets of the West End, parts of Kensington, and near the Tower of London, are another menace. Often trading out of cardboard boxes, milk crates, or suitcases, they specialize in shoddy or counterfeit goods.

The most popular items this summer are watches - such as bogus Piagets, Longines, and Cartiers - Lacoste sport shirts, ''gold'' jewelry, and cut-price perfumes. The latter, wrapped to resemble famous French brands, are, in fact, only colored water.

Cassette tapes offered at apparently knockdown prices come complete with pictures of famous pop groups, but more often than not the tapes turn out to be blank.

The gangs are slick operators. They set up lookouts so they can make a fast getaway if ''the law'' appears. Initial sales are made to members of the gang, to encourage legitimate shoppers to gather and buy. Then the salesman, armed with a persuasive and often witty line, takes over.

The turnover is fast. It is usually only when the victim returns to his hotel that he discovers he has been duped. By then, there is little he can do but complain. It is unlikely that the culprit will be caught, because few ever work the same pitch two days running.

Police and local authorities, who share responsibility for prosecuting sidewalk peddlers, carry out frequent purges. But they admit it is generally only a matter of days before the racketeers return.

Under present government constraints, the authorities have neither the staff nor the resources to get illegal traders off the streets altogether. Even when they bring a case to court, the fines imposed are a fraction of a trader's daily earnings.

A government official commented wryly: ''They look on the fines they pay as overhead - not as a deterrent.''

A spokeswoman for the London Tourist Board, Ylva French, explains: ''The magistrates seem to take the view that these traders are basically nice people who are out of work and trying to earn a living. They don't see it as a crime, and as a result, police tend to feel there is little incentive for them to act.

''But we're not talking about nice people,'' she continued. ''We're talking about a racket.''

An equally insidious trap is the mock auction conducted in shop premises rented only briefly.

Fast-talking operators draw shoppers inside the would-be auction house. Gang members planted among the bidders push prices up. But the items displayed bear little relation to those the buyer is given, wrapped and sealed, when he hands over his money. Many an ''onyx'' bowl has turned out to be painted plaster. Legislation to wipe out mock auctions is being drafted.

Phony photographers who corner visitors outside well-known tourist attractions are another source of concern for the London Tourist Board. Legitimate photographers are issued licenses and can be recognized by special armbands.

The board is eager to point out that the exploiters are very much in a minority. But Ylva French advises visitors to make sure they know what their money is worth before they arrive, to use black taxis, and to stick to recognized street markets, such as Portobello Road and Covent Garden, where stallholders are licensed.