Well now that is different. More letters are worth more than others as well as the extension. Long term value? 3 years ago I would have said yes but with the new TLD's being fired out and the eventual "word TLD's" like .microsoft and .facebook I am not sure. Some letters have more values than others. If you had aaa.co.uk or similar god yes it would be worth. or sex.co.uk. If it is a word you will bank hard
Random letters on the other had it is hit or miss. same with letter letter number. If you sat on it until it is about to expire and can push it to a potential buyer you might get double what you paid for. it is really hard to say.
I used to follow the domain market awhile ago but couldn't get into it. I learned what I know (which is not that much) from digitalpoint. You can look there in the domain section