I suppose Little Chefs are the nearest to the kind of off-motorway roadside restaurant chain, though I can't recommend them wholeheartedly from a culinary point of view. But I think you're right about that culture not (or barely) existing: I suppose the idea is that most people would find a pub for lunch. That said, in Wales in particular I've sometimes found roadside cafes (invariably with homemade bara brith) that have been pretty good.