LONDON -- Britain gained a new first daughter-in-law this weekend, as Mark Thatcher, 33, married 26-year-old Texan Diane Burgdorf in a Valentine's Day ceremony here.

The bride is the daughter of wealthy Dallas car dealer Ted Burgdorf, who described her as "just an ordinary millionairess." The groom is a one-time accountant, racing driver, international consultant and current auto sales representative in Texas -- and, of course, the son of Prime Minister Margaret Thatcher.

The Church of England ceremony, attended by about 150 family members and friends from both sides of the Atlantic, took place in the 16th-century Savoy Chapel in central London, a small building that also hosted the nuptials of the poet Chaucer and is mentioned in the 17th-century writings of famed English diarist Samuel Pepys.

The setting apparently was chosen, however, not for tradition but for security reasons. Once it was determined that the prime minister would be unable to fly to Dallas for a wedding in the bride's home town, the chapel was selected as the closest place to the prime minister's residence in Downing Street and the reception around the corner at the Savoy Hotel.

Roads around the area were blocked off, and police marksmen stood ready on nearby roofs; security personnel far outnumbered the handful of onlookers who braved the wet and chilly Valentine's Day weather for a glimpse of the newlyweds, the prime minister or the attending Dallasites. Many Britons are on intimate terms with J.R., Pam and Miss Ellie, and have a strong curiosity about the real thing.

But Mark Thatcher's wedding did not seem to capture the public imagination here. Perhaps the masses paid heed to the couple's pleas for privacy, communicated over the past few weeks by sullen glances and sharp words to pursuing photographers. Mark Thatcher's distaste for the main chroniclers of celebrity events, the British tabloid press, is reciprocated. Front-page photos of the wedding party, in which the groom barely managed a grimaced smile, were captioned "Bride and Gloom." The most visibly happy person at the event was the prime minister, radiant in a blue dress and hat, and seen by waiting reporters to be nearly overcome with emotion. With her only other child, Mark's twin sister Carol, still unmarried, Mrs. Thatcher has expressed a strong interest in Mark's quickly making her a grandmother.

The prime minister is known to dote on her son. During her 7 1/2 years in office, he has required considerable doting as he has struggled to find a career, been repeatedly accused of trading on his mother's name, and generated bad press for her and her political party. As she has protected and defended him, he has appeared to many commentators to be the one emotional chink in his mother's otherwise impenetrable public armor.

After graduating from Harrow, one of Britain's leading private schools, Mark eschewed university training and eventually signed up as a trainee accountant in London. Four years later, in 1977, he reportedly failed to qualify for a staff job and left to join an Australian freight company. In 1979, the year his mother became prime minister, he became an international "consultant," lending his suddenly celebrated name to the promotion of whiskey and sporting equipment, and working as an occasional racing commentator.

His racing career began about the same time, and he built a respectable reputation that culminated in 1981 when he was lost for six days in the Sahara Desert during a cross-country rally. His disappearance sent his father Denis, a wealthy businessman, to join the search operation from Tamanrasset in southern Algeria, and caused his mother to break down in tears at a public function.

While cultivating a dashing reputation, however, Mark continued to live with his parents at No. 10 Downing Street. His romances, particularly a six-month fling in 1984 with another Texas heiress, Karen Fortson, were regularly publicized as they flamed and then fizzled out.

His business career, described by one chronicler several years ago as that of an "upmarket sandwich board," came crashing down in 1984, when it was revealed that he had been involved in a lucrative contract awarded by the sultanate of Oman to a British construction firm.

When the contract was signed, in 1981, it was attributed to Mrs. Thatcher, who had actively promoted the deal on an official visit to Oman. When it turned out that Mark, as the firm's paid representative, had been in Oman at the same time as his mother and had met with her on the trip, the prime minister was accused by angry parliamentary opponents of a conflict of interest.

As is her custom, Mrs. Thatcher chose to stonewall. "Members of my family are as much entitled to their privacy as any other citizen in the United Kingdom," she said. She refused to comment on allegations that she had served to benefit the interests of her son.

Although the controversy eventually quieted, it was too much for Mark. Announcing that he was no longer prepared to tolerate the intrusion of the British press into his life and was unwilling to cause his mother further political distress, he left for the United States and a job as Dallas-based "public relations adviser" to Lotus Performance Cars.

His recently announced engagement to blond, blue-eyed Diane Burgdorf after a two-year courtship sent a phalanx of tabloid reporters to Texas. It was reported that Diane was the perfect "Barbie-bride," a typical representative of the cheerleader-variety Dallas millionaires' daughters who like to use their daddy's credit cards and shop at Neiman-Marcus, a place one British reporter found "a disappointing department store."

The $600,000 house where the two plan to make their home in a Dallas suburb has been pronounced wanting, and bearing little relationship to Southfork. "The 'grand staircase,' " wrote a reporter who managed to talk his way past construction workers, "is 18 wooden steps with knobbed wooden bannisters down which Sue Ellen wouldn't be seen crashing, even dead drunk."

But despite some disdainful references to the fact that she bought her $3,000 wedding gown off the rack at Neiman-Marcus, the British media has been relatively kind to Diane, implying that sympathy is in order for anyone destined to spend a lifetime with Mark. It has even treated as quaint her mother's reply to a question on the couple's premarital sex life. "Good Baptist girls don't go in for hanky-panky before they get married," Lois Burgdorf responded cheerfully. "Get what I mean?"

No such tolerance has been allotted to Mark. Far from generating the acclaim of a prodigal son, his return to London several weeks ago was an occasion to dredge up all the bad news from the past. Responding in kind, he has been crotchety with the media, refusing to smile and behaving, in the words of one reporter, "like a cross between Greta Garbo and a Russian dissident on the run."

The somber couple left London Sunday morning for a honeymoon in Australia.