From 1872, 1982--whenEngland built a residence in Washington for Queen Victoria's minister--until the late 1920s, British embassy employes received tropical duty pay because of the region's heat and humidity.

Recently, 23 residents from the historic borough of Runnymede, England, experienced for themselves the wilting weather conditions that compelled their countrymen to consider Washington a hardship post.

The British group was here for a two-week cultural exchange with residents of Herndon, Runnymede's sister city in the United States.

"It's rather like an Amazonian rain forest," Jean Gunns, a teacher and potter in Runnymede, told her Herndon hosts, attorney Tim McPherson and his wife, Karen, a research assistant.

On the whole, however, the British visitors seemed to have, as they would say at home, "a smashing time."

In the first week of their visit they were treated to a thunderstorm (their first night here), a champagne brunch, a reception, a picnic, a pool party, several dinners and tours of the White House (courtesy of Republican Rep. Frank Wolf), Washington's museums and monuments, a suburban shopping center and the Virginia countryside.

The pace left some of the visitors a little breathless.

Harry Hobbs, a retired British postal worker who has traveled in this country and Canada with his wife, Lilian, said he feels his countrymen, who are used to having things "nicely organized," may have found the "slapdash" pace of American life "just a bit difficult to cope with."

The visitors had several free days, however, before a farewell party in their honor.

Although they are on opposite sides of the Atlantic, Runnymede and Herndon share a number of common traits, which led to their being "twinned," as the British say.

* Both are commuter communities composed primarily of middle class residents.

* Both are about 20 miles west of their national capitals, and as a result, both suffer the torments of rush-hour traffic.

* Both are near international airports, Dulles and Heathrow. (The British visitors used neither: They took off from Gatwick, about 30 miles from Runnymede, and landed at Baltimore-Washington, about 50 miles from Herndon.)

* Both are near famous rivers, although Runnymede has the edge since the Thames runs through the entire length of the borough.

Runnymede, of course, is a bit more celebrated than Herndon, since it was where King John bowed to his rebellious nobles and signed the Magna Carta in 1215. At the time, Runnymede was a lush English meadow. It did not become a borough until 1977. Herndon, on the other hand, was first settled in 1790 and was incorporated as a town in 1879.

With so many things in common, it is hardly surprising that the British visitors and their American hosts got on famously.

"(The British) are delightful people," said Patsy Rust, wife of Herndon Mayor Tom Rust. "I never have been one to have someone live with me that I did not know but they just could not be nicer--they sort of became members of the family."

The visitors, for their part, seemed delighted with their hosts and Americans in general.

"People are very informal and friendly and we are having great fun," said Julia O'Keefe, who is hospitality secretary of the Runnymede group and works for the British branch of M&M Mars Inc., the candy makers. "The other fascinating thing is the number of people we've met who are of different nationalities--Norwegian, Irish . . .--but they are all so American."

"Still," said her husband, Ned, who works in computer training, "they all have a good idea about their national origins."

The O'Keefes planned to "hire" a car to visit the M&M Mars plant in Elizabethtown, Pa., as well as Gettysburg and the Amish countryside.

Several of the visitors were amazed at the resemblance of Herndon and the Virginia countryside to their own home. "(There are) lots of small roads that are very like England," said Jean Gunns, "until you come to the houses or roadsides."

Gunns also was impressed with the American capital. "Washington is beautiful," she said. "It reminds me of Paris."

Gunns' husband, Jim, is chairman of the Runnymede Twinning Association, which sponsored the British group. Next year, Runnymede plans to host Herndon visitors.

The British visitors paid their own air fares, and the cost almost kept the vice chairman of the group, Maud Austin, from making the trip.

But Austin, who helped start the twinning concept in England 21 years ago, said she decided to take the advice of a friend, who told her, "Live now and pay later!"

There were some problems for the British and Americans over language differences.

Margery Key, longtime secretary of the Runnymede Twinning Association, said the differences provided "a few laughs, actually some that you couldn't print." Breaking into laughter, she declined to elaborate other than to cite the common example of the English phrase "being knocked up in the morning." In England, of course, that means being woken up.

And Key, whose husband, Stuart, is a retired British army captain and insurance broker, said she and the other Runnymede residents "love being called 'you all.'

"Now we call ourselves the Runnymede You-alls!"