I've done it twice.
My recommendations:
Go south to north (LE JOG) - this way, as Marco says, you have the prevailing wind behind you, and you get the worst hills (seriously, those in Cornwall are short, sharp little "characters") out of the way early.
Plan on between 70 and 100 miles each day. This way, you'll enjoy the holiday and have time to see the scenery rather than just blasting along.
Join the CTC, again as Marco says, there's loads of information there.
Pack light. I'd go with YHA and B&B accommodation rather than camping. There's plenty of it about, and you can cut your load down by about 50%.
Take maps. Or at least cut down a decent road atlas.
Take Chamois cream. And several pairs of shorts.
Do a couple of weekend touring trips before the big tour. Get used to how your bike feels loaded up and the "rhythm" of bike touring.
Get a low gear.
My favourite route:
Train to Penzance
Penzance
Lands End
Bodmin
Crediton
Bridgewater
Bristol
Gloucester
Evesham
Stratford
Tamworth
Burton
Newark
Lincoln
Humber Bridge (I like suspension bridges
)
York
Northallerton
Durham
Hexham
Jedburgh
Edinburgh
Forth Road Bridge
Stirling
Callander
Crainlarich
Bridge of Orchy
Ballachulish
Fort William
along the Great Glen
Muir of Ord
Conon Bridge
Alness
Golspie
Helmsdale
Wick
John O Groats
Wick
Train back to London.