No tour suggestions, but:
Buy extra film, envelopes and stamps. Mail the completed tapes home (may get a bit expensive) and you wont have to worry about them getting lost/broken/wet/whatnot.
The lakes ditrict is nice and the whole South coast. Scotland is also amazing.
With the time you have available you have more than enough to walk from Land's End to John O'Groats (over 1000 miles) with stop-offs and side trips to the best scenic, urban and rural places en-route. The walk, apparently, takes about 10 weeks plus a couple of weeks for rest en-route.
The Cornish coast is well worth seeing.
The Lake District is superb, but there's often too many tourists.
North Yorkshire moors are well worth a visit if you like 'bleak'. Some say this area is even more bleak than Dartmoor.
A side trip to Ireland would be well worth considering.
I've never been to Scotland (except on a business trip) so I couldn't really comment!
Once you get up to the Midlands you can hop skip and jump up north via the national parks, if you like rugged scenery dont miss the west coast of Scotland, theres some pics of the U.K (look in the walks section) on my site, feel free to browse.
I just got back from a trip to the UK, although I stayed in London and Edinburgh for most of it. Not much for a walking tour there.
Although there's a lot of great English countryside, I'd definitely recommend heading up to Scotland. The views are amazing, and there's a ton of hills and mountains to climb. Lochs (lakes) are everywhere as well, if you want to throw some boating into your walking mix.
As for particular places, you might want to head toward Ben Nevis - the highest mountain in Scotland (and the UK). You might also want to head towards the Glen Coe, a valley in the area around Ben Nevis. It offers some great picturesque views, as well as a place in history as the sight of the "massacre of Glen Coe."
I've been to Ireland once, but it was five or six years ago. There are definitely some nice scenic areas to hike and travel through, but I couldn't remember the names of any of them. You probably couldn't go wrong if you just hiked in a random direction.
Good luck, and enjoy,
London is too crowded and boring. There are many castles all over Great Britain that are interesting. Tenby in South Wales has the best beaches in the summer(?!). Alton Towers is expensive, but if you go during school terms the queues won't be so big.
For historical value try:
Black Country Museum
Windsor Castle (it massive!)
The best thing to do is try and schedule your travels with events that occur in certain places at certain times.
Edinbrough Festival (lots of entertainment) - August
Abbots Bromley Horn Dance (lots of drinking) - early September
Glastonbury Festival (lots of mud) - June
What are your interests?