It was 11 degrees out there yesterday morning at the National Gallery's East Building as the line of about 200 bescarved, besweatered and beparkaed esthetes and anglophiles snaked around the corner of Fourth and Pennsylvania, in place for the doors to open on "The Treasure Houses of Britain."

Gallery officials said they had feared that the morning's weather, which some broadcasters were calling the severest cold since President Reagan's inauguration, would undercut the holiday deluge they were prepared to handle for their treasured "Treasure" show.

It didn't.

Washingtonian Tom Balsonek, a tall and apparently hardy individual, got there first. "We had been planning to come for a long time, and my mother-in-law got me out of bed hours ago to come down this morning and get the passes so we could come back in the afternoon" -- all seven of them. "We learned the hard way a long time ago how you have to plan ahead if you want to be sure to see these things."

About 20 minutes before the 10 a.m. opening time, the front door guards took pity on Balsonek and the first 50 people behind him and let them into the lobby, to line up near the window where the free passes are issued.

There had been kids playing in the revolving doors all along. "I let all babies in, and the elderly and the handicapped," said William Baytop, operations chief for the guards.

Because of the cold, things may have started a little slowly yesterday morning -- by comparison with what was anticipated -- at this remarkable extravaganza of objects from England's country houses. By any other standards it was already a mob. Quite aside from the majority who stood in line for passes, there were hundreds of reservations, some for small groups and others for bus loads. Said one aide, "It's beginning to seem like we emptied out the city of New York today."

By limiting the number of passes per day and pegging them to certain times of the day, officials control the flow of viewers through the show. They reached "capacity" yes- terday at about 1:30 p.m., at 7,500 passes, meaning that everything was sold out through the 7 p.m. closing time.

The definition of what constitutes "capacity" will change from day to day, depending on how the flow is moving. For instance, on the other busiest day since the exhibition opened in early November, the Friday after Thanksgiving, "capacity" was 8,000, but apparently the longer pre-opening line filled up the early hours as well as the late.

Some days the exhibit does not fill up. Early December was slow, according to gallery information officer Neill Heath, "and early January probably will be too," he said. One lucky observer showed up in late afternoon on the day of last week's storm and had the place practically to himself.

Heath estimates that 300,000 people have visited the "Treasure Houses" exhibit since its opening on Nov. 3, in the presence of its royal patrons, the prince and princess of Wales. The exhibit is scheduled to remain through March 16.

The idea of passes originated with the "King Tut" exhibition that came to the National Gallery from Egypt in late 1976. At first people simply came down to the gallery and stood in line, taking their chances. The result was jammed galleries and impossibly long lines, with substantial numbers waiting in vain. So midway into that show, a pass system was instituted.

Yesterday's mob was almost unfailingly cheerful, and the flow seemed excellent, partly a result of the spacious installations but also a byproduct the nature of the "Treasures" exhibit. It is not a bunch of bam-bam blockbusters, around which layers of lookers linger regardless of the efforts at crowd control. Instead, it consists of a plethora of objects -- all valuable and interesting -- that are of such astonishing variety that they are, collectively, a blockbuster.

Yesterday's only real bottleneck, even at the peak of the afternoon, was at that case of gold, diamond, emerald and pearl tiaras near the end. It is pretty dazzling. If there was an underlying theme to the crowd, it was family day. There were strollers all around, and the children were notably calm.

There was one family from Bethlehem, Pa. The mother cracked, "Since it's Christmas, everybody is supposed to stay up there." Daughter Ann, who is 6, expressed misgivings. "My mommie dragged me down here to see the state bed." But Ann agreed to come along on the condition that she come with her "Real Baby," which appeared under the tree Christmas day.

Another thing one noticed was how knowledgeable people seemed to be about the immense historical import of some of the objects. One young woman was explaining to her parents before the state bed, "One of the countesses on the TV show [the current PBS series on English country houses] was explaining, 'Well, we just found this in the attic.' And she said it as if to say, 'well why don't we have a rummage sale.' "

Almost universal was the informality of the garb, maybe a little different from the way Americans would once have approached the trappings of royalty.

Even the august Joseph Alsop, author of "The Rare Art Traditions: The History of Art Collecting & Its Linked Phenomena Wherever These Have Appeared," was seen in open-necked dark blue shirt as he guided some young friends through the show.

The prevailing dress -- and, for that matter, the whole character of this fresh and matchless exhibit -- brought to mind a classic story about the Duke of Devonshire. He is the lord of Chatsworth -- one of the greatest of the country houses represented in this show -- and one of the true lions of the British aristocracy.

The duke was asked by somebody at Chatsworth why he chose to dress so, shall we say, mustily for someone of his station.

"Well, you see, it doesn't matter," the duke is supposed to have replied, "when I am here everybody already knows who I am, and when I am in London, nobody does."

Right on.