She will be as old as the century on Aug. 4, but Britain's beloved "Queen Mum" still cuts a lively figure on the country's cultural scene. And, as a new 12-pence postage stamp plainly shows, a fine-feathered figure on the country's envelopes. The stamp, the first ever honoring a dowager Queen, is part of a royal flush of tributes planned for the Queen Mother's 80th birthday. This week she will be feted for three days in her capacity as Lord Warden of the Cinque Ports. The 900-year-old post, always held by a man until she assumed it last year, no longer requires its bearer to fight England's sea battles. It does, however, give Elizabeth sovereign beachcombing rights to anything that washes up on England's southeastern shore. Birthday greetings in bottles, for instance.

No more rubber chickens tossed at needling journalists in the briefing room. No more Frisbee games on the lawn of Anwar Sadat's summer home. And, alas, no one to lead those rousing choruses of "I'll take the West Bank and you take the East Bank" on peacemaking missions to Jerusalem. For when Nodding Carter resigned his post as State Department spokesman early last week, the Administration lost one of its freer spirits. Naturally the boys in the State Department Correspondents Association were not about to let him off easily. Midway into a final noontime briefing, in walked a singing messenger to crown Carter with a dunce cap and bid him a musical farewell (sample lyric: "We cry our eyes out every night./ Not because we miss you so,/ but because you didn't go/ even sooner than we'd hoped you might"). Said Carter, diplomatically: "I don't know who the jerks are who did this, but I thank you."

If you liked Lynda Carter in her Wonder Woman suit, you'll love her in her next TV special. The show,set to appear on CBS this fall, features lissome Lynda in nine costumes, all tres Cher by Designer Bob Mackie, and each more wondrous than the last. In a tribute to sizzling Tina Turner, Carter dons flame-red tatters and a ginger wig and makes her entrance on a mock wheel of fire. Paying homage to Bette Midler, she slithers down a giant banana, ablaze in golden curls and boa. Perhaps most superheroic is the kiss she blows to the hard-rocking band Kiss: clad in feathered headdress and see-through black net, the sometime woman of steel makes a 20-ft. descent on a huge metal spider web. Top that one, Spider-Man.

Move over Fat Albert and the Stoned Lone Ranger. In his latest picture Comedian Bill Cosby plays his baddest dude yet: Barney Satan, left-hand man to Old Scratch himself. The diabolical plot of Disney's

The Devil and Max Devlin involves much wheeling and dealing between Cosby and a hell-bound Pasadena slum landlord named Devlin (Elliott Gould), who is offered a rebate on his soul if he can persuade three others to mortgage theirs. The movie was shot in Hollywood, and Cosby has been cutting quite a figure there in his devilish outfit. Says he: "My wife was the only one not surprised by it. She says I look like this 30% of the time."

On the Record

Jack LaLanne, fitness buff, declaring that he may never die: "It would wreck my image. I cannot even afford to have a fat dog."

Mother Teresa, of Calcutta, Nobel-prizewinning missionary, asked if she does not become discouraged in her work with the destitute and dying: "God has not called me to be successful. He has called me to be faithful."

Eric Heiden, Olympic speed skater: "People ask me to give speeches. I'm 21 years old. What can I tell anybody?"

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