In reply to dr6:
I take hundreds of people up and down Scafell Pike every year. A common complaint is ankle / knees pain and the effect loose boulders / scree has on the joints, and on general ability to balance. The only way to train for this is to do it and from what I can gather that is impossible in or near to London. So next best thing:
Up and down lots of stairs with a backpack on will make for a good start.
Lots of time on your feet will also be of benefit.
Getting your general level of fitness up will be essential if you're hoping to enjoy it.
Going to the gym may help a little, but walking as much as possible would benefit you more, the rougher the terrain the better, as Scafell Pike terrain is rough by most peoples standards. The paths are not as good as those on Ben Nevis and although the height gain is less for Scafell Pike, don't let it fool you into thinking it's easy by comparison, it's not!
And if you don't feel ready for the hike just yet, why not put it off for a while and use the time to walk some of the smaller mountains, build up your mountain fitness, that way you may well enjoy the experience much more.
A few photos from our site, may be of interest:
http://www.climb-scafell.co.uk/gallery.html
Hope this is helpful