Vegetarianism has become more widespread in the UK over the last few decades. If you are staying as a guest in a British home it would be considered courteous to inform your host beforehand as to any dietary requirements, but this will not be considered rude or even particularly unusual. If you are staying in a B&B, let the owner know when you arrive, and you'll often find that they will cook up a special vegetarian breakfast for you.
Bear in mind that even if you call yourself vegetarian some people will assume you eat fish, so if you don't, then tell them so. Nowadays, it is rare to find a pub or restaurant with no vegetarian options.
If you are a vegan, be prepared to explain precisely what you do and don't eat on a fairly frequent basis. Outside of specialist eateries, most places probably won't have a vegan-friendly main meal, so be prepared to hunt around, order bits and bobs, or in a pub make do with the ubiquitous bowl of chips and tomato ketchup and even then it would be wise to check whether the chips have been cooked in animal fat, a practice quickly falling out of fashion.
In general, the best places for vegetarian and vegan food are specialist veggie pubs and restaurants and Indian, Chinese and South-East Asian restaurants. Most major cities and towns will have at least one. Expensive upscale restaurants may have more limited vegetarian options, and sometimes none at all. If you're fortunate enough to be dining in such a place, it may be worth ringing ahead.