I'm thinking of possibly building a guitar, and I need somewhere to buy the wood for the body to start the process.

Does anyone know where is a good website, shop or location to buy either Maple, Alder or Mahogany. Possibly Walnut
I'm afraid the UK doesn't have too much in that respect, but, luckily, several US companies wills ship to the UK.

tonetechluthiersupplies.co.uk is (obviously) UK based, and sells that sort of stuff.
stewmac.com will ship to the UK, as will luthiers mercantile (llmi), and rctonewoods. I've dealt with the last one, and would highly recommend them, although I don't know if they sell electric guitar body blanks.
Quote by Kensai

Awesome guy right here
As previously stated, it all depends where you are in the UK, it's a big place.
Personally, I buy all my guitar building timber from Gilmour and Aitken, of Jamestown, West Dunbartonshire. They are the largest importers of hardwoods in Scotland. Many top guitar builders are supplied by them. MOON Guitars to name one. Huge stocks of hardwood. I love going there and hand selecting timber for my builds.