Ditto much of above.
Decide now if you are going to be camping or hostel/ b&b's. That will determine bike choice to a degree, tourer for camping ,road bike otherwise.
I strongly advise you get a triple...I did it on a compact and trust me there are HILLS in the UK.
I'm not sure why many peeps seem to get so hung up on the speed it takes them to do it, unless you want a place in the record books this is pretty irrelevant (work comitments aside i guess). Choose a route that takes you through the nice bits of this greena nd pleasant land, what is th epoint in busting your gut shooting up am lot of A roads just so you can boast how fast you did it?
Take a good camera, and stop when you see a shot, remember you wont be back there for a while so dont ride past it just because you are on some kind of deadline.
Get some proper road training in, it's totally different to the gym.
Learn how to fix your bike, at least the basics.
Get the right clothing.
Get the right maps, I used a GPS with pre loaded track files for each stage.
If you ride unsupported on a road bike carry a rucksack unless you can get a rack fitted for some small panniers. I rode with a rucksack against popular advice and it was fine.
Take some pain killers