I was under the impression that, since U is not a pass grade, it doesn't count as a qualification and isn't certificated; at least it didn't used to be. Whether that means you don't have to declare it I don't know, but I would have thought that, since you don't have it as a qualification, you don't have to declare it. The answer pretty much lies in whether or not it's on your GCSE certificates. If it is, then you'll probably have to declare it; if not, then you won't. If all else fails, phone UCAS (but maybe wait until September because they're busy enough as it is).