A few books to track down: I have the US editions, well worth the money and all save you time and money! Great for first-timers, and even though I've been going to Disney parks for over 40 years now, I still buy new editions of them and use them.
(Amazon UK links)
What to do other than the House of Mouse
Beyond Disneyand one for the Magic Kingdom:
Unofficial Disney Worldand one for those with young Mickey Mouse fans in tow
Disney With KidsNote that all of these books have new editions coming out in April of this year, maybe before you leave, but probably not. I would see about getting used copies, but only if you can get ones which aren't marked up or have torn-out pages. Don't get any book over 3 years old; the parks change attractions on a regular basis, and you might have your hearts set on seeing/riding something which no longer exists.
I note that the Amazon UK site lists books by Brits to Disney parks, but I have no knowledge of those editions. You could look them up anyway. There are links to them on the Amazon pages above, especially the third book for children.
Don't get any of the Birnbaum books, Disney subsidizes them and they are not impartial when it comes to certain things Disney. Borrow them or get them used; they're good for reading along with the ones above, but not as your only planning books.
I also highly recommend one of the behind-the-scenes tours, like the Keys to the Kingdom tour.
Here's the thread - warning, contains spoilers to the tour! Ianflies recommended it to the rest of us. There are other tours if this one doesn't suit; you learn a lot, have a good time, and get to see things you wouldn't ever see in a regular day at the parks. There's a thread below Ianflies' about another tour you might consider.
Also, if you see a copy of Disneyland Detective,
this Disneyland book for sale, get it while you're in the US. Most Borders books and Barnes and Nobles have it, Amazon UK doesn't seem to. The link is to Amazon US. It covers Disney history, and things like 'Hidden Mickeys' in the park. A good read, even if you're not going to CA.
Have a great time, and remember this: you can not do all of the Magic Kingdom in one day! Don't even try.