Traffic police patrol cars are "jam sandwiches" because they are white on top and the bottom with a red stripe in between. London's North Circular Road, a congested version of the Capital Beltway with traffic lights, is called the "Big C." Police traffic wardens are "custard" (male) and "custard tarts" (female).

Along with "10-4" and other familar jargon borrowed from the United States' cluttered citizens' band air waves, this is the language of an outlaw medium that has become Britian fastest growing new mode of communication.

An estimated 30,000 CB radio sets are in use here, and hundreds more are being smuggled into the country each week.

But the CB radios are all illegal, and their operators are "trip-takers": breaking the law. Although citizens' band radio is permitted in almost every other country in both Western and Eastern Europe, it is still banned in Britain.

Many British government bureaucrats believe CB is antisocial. They point to the crudity of CB talk in the United States, to the use of CB to evade enforcement of speed and truck-weight laws, and to the interference CB causes other radio signals.

In Britian, radio-controlled model airplane hobbyists direct their big, expensive, painstakingly consturcted drone planes by the same radio band that carries illegal traffic from U.S. made CB sets. Model planes worth $1,000 and more -- not taking into account hours that go into building them -- have been shot down by CB radio interference.

It is quite unintentional of course, but the Society of Model Aeronautical Engineers, representing an estimated 7,000 British model plane builders, is up in arms. They want outlaw CB broadcasts stoped.

Bureaucrats in the British Home Office responsible for both use of radio waves and law enforcement here, recently sent the police around late at night to question CB users and search their homes for illegal sets. One man was briefly jailed.

Meanwhile, the Citizens' Band Radio Association, which has been lobbying the legalize citizens' band in Britain, has come up with a plan it contends would make everybody happy. With the support of a group of Conservative and Labor members of Parliament, it is urging the Home Office to permit the use of certain FM channels for personal radio. CB users here would then communicate on FM with Brithsh-made sets specially designed for limited-range FM broadcasting.

Citizen Band Radio Association President James Bryant and his supporters contend this will prevent interference with model plane flyer or anyone else, would make British CB users easier to license and regulate their range of broadcasting, and would boost British industry by creating a market for new personal radio sets for several hundred thousand Britons.

Otherwise, CB enthusiasts here warn, illegal use of U.S. made CB set will grow so rapidly that Britian eventually will be forced to legalize transmissions on the same frequency as the planes, as Belgium and the Netherlands have done. Chaos on the air waves and mass destruction of radio-controlled model airplanes would then be inevitable, according to Bryant, who pointed out that a 10-pound model plane flying at 100 miles an hour can kill someone if it crashes.

On the other hand, CB enthusiasts argue, personal radio could save lives, as they say it has in road accidents and natural disasters in the United States. One pro-CB member of Parliament, John Butcher, said he became interested in the usefulness of the medium when he watched cousins in New Jersey and Texas round up cows using their CBs for communication.

Butcher and Bryant believe they are making headway with prime minister Margaret Thatcher's Conservative government. Butcher and his all-party parliamentary committee on CB legalization are scheduled to discuss the subject with government ministers again soon. He hopes to have a government statement supporting the use of FM channels in principle before the end of November.

Home office spokesmen will not comment on that. They still sound distinctly frosty about the idea and refuse to discuss their end of the negotiations.

Bryant sais Butcher and many other Conservatives are on his side because of their belief in personal liberty. They do not think a Conservative government should interfere with anyone's freedom to communicate, he said.