Sure, their productions are terrific. Producer-actress Sue Birch and her loverly casts get giggly with British bedroom farces and raise the roof with shows such as the sexed-up Shakespeare they did last season. But adding to the big fun at Theatre Britain's stagings at Trinity River Arts Center are the delicious British intermish noshes. Brought over from the British Emporium in Grapevine, the snacks include English tooth-rotters such as Maltesers, Flake, Crunchies, Mars Bars, Cadbury's Buttons, Smarties, Wine Gums, Refreshers, Fruit Pastilles, Fruit Gums and<\f>Walkers Crisps (Plain, Salt & Vinegar, Cheese & Onion). So good are the edibles, sometimes the crunching threatens to drown out all Act 2 dialogue. But go ahead, munch away through Macbeth. Those salty crisps are worth the Thane of Cawdor's nasty looks.