Was going to post this last night but some moron decided to have a go at hacking the site

+1 to who ever mentioned the Lake District.
Sheffield is not a bad base if you want to spend some time in the middle of England with a number of touristy places near by and Leeds (which is home to several attractions of it's own) within reasonable travel time and several major Airports within reasonable distance, the down side would be that Sheffield to London is a bit of a trek.
I would also suggest a day or two in and around Wales, they have some spectacular views around there, just watch the weather if you planning on going up the peaks. If you are out that way then a hop across to Ireland might be worth your time as well.
Scotland also holds it's own for attractions, (The Edinburgh Military Tattoo is a particular interest of mine that I would love to go to see). The only thing I will say about Scotland is pick your time of year, it's far enough north that the weather can be interesting at the wrong times.