In reply to astraeus:
I've only been to a few of the peak district crags but of the ones i have been to you dont need to be able to abseil to climb there - you can usually walk down from the top.
As for learning how to abseil, I personally watched a few instructor videos on youtube, then found a hill with a tree on it and practised my technique there (the idea being if i mess it up i wouldnt fall and die!).
I try and avoid abseiling if i can as its apparently the most dangerous part of climbing as you can potentially get hit by rocks you may dislodge on the way down, and get knocked out etc.
Not sure about where to get lessons to abseil but as with learning to trad climb, a great place to start would be to join a local climbing/mountaineering club - there will be loads of people with loads of knowledge who im sure would be happy to help you with all aspects of ropework.