Congratulations! I'm sure you'll have a wonderful time! I've competed at the British Open and it's such a treat to see so many people and such fine dancing, day after day (after late night after late night...).
I don't know how the UK championship may be different, and I don't know where you're from, but I'll tell you what I found. Practice: there's the Tower ballroom that has a nice floor and organ music, open for dancing fairly often (but not all the time; be sure to check ahead) for a small fee. It is social dance time, so be considerate of the regulars! There were practice spaces reserved for competitors from certain areas (for example, the North American competitors' space is in a hotel).
There are times for practicing on the floor itself, also a bit structured if my memory serves. Check on that when you get there. The floor is crammed though, so use it to feel the floor and get used to the hall. It won't be a great dance session, just a quickie to feel comfortable in the space (and what a space the Empress Ballroom is to dance in!). When we were getting coaching it would be in a studio or the practice ballroom, so we never asked about renting floor time. Anyone else know about that?
Blackpool is crammed with B&B's and hotels - that's pretty much the whole town! B&B's are much cheaper of course, and are all around the WinterGardens, so many are an easy walk away. Our favorite place was the Goldhurst, but I'm sure you can find a good place (tourist info can recommend and even book rooms if you tell them the requirements - area, price range, sky TV, private bath, etc.). Lots of them are listed in the internet too.
You don't have to rent a car if you think you'll be happy in Blackpool. You can take the train and then taxi to your B&B. It's a short walk between the WinterGardens, the Tower, waterfront, pedestrian shopping area and lots of stores. On the other hand, it's not the most... um... cultural town, so if you want to have a country getaway day you may want a car for that. Or you could take the train or buses, go to Liverpool or something like that.
Yes, you can get lessons from coaches there, though it works best if you contact them before you go, since they will likely be very busy. Pay after the lesson, unless you want to get it out of the way and pay before.
Have a great time... fill the space with your dancing!