BRIT, the Museum of British History, tries hard to combine tradition with modemity. Tonight there's to be an extravagant banquet to celebrate the opening of its much touted exhibition ELEGANCE: guests will dine on Lobster House of Stuart, Beef Plantagenet and Berries Tudor Rose. Present will be a wayward chef, an overambitious Director, a vainglorious Chairman, a restored Gainsborough portrait of a lady dressed as Puck...Add backbiting, sexual Innuendo, even romance - it's all highly suitable for a Midsummer Night. And just what is wrong with that Hound in the Left-Hand Corner?