Lets not forget either that most AA's are charity funded, with some struggling to meet the flying task with the money that comes in. As Louis discussess, the additonal engineering requirements, the more limtied flight envelope (C of G issues) and often shorter winch wires, the extra training and the risks associated with winching would add a considerable financial burden to any AA charitable organsation even if they were cleared by the CAA to do it. Its simply not worth the effort in most parts of the UK when there are perfectly competent SAR winch platforms at immediate readiness not far away. Lets not forget either that AAs are day only too.
It is a noteable however, that as AAs have proliferated in recent times, there has been tendency to send an AA first sometimes when the immediate tasking of a SAR cab would have been better. This of course comes down to what information comes from the accident scene and how the control room responds to the request. While it is also true that SAR cabs have been tasked to what would have been better done by a cheaper AA, from a casualty's perspective that error (from a time saving perspective) is better than having to wait while an arriving AA asseses that there is a need for a winch platfrom.
Much has been done in recent years to get good working relationships and coordination going..particularly at the tasking level...but there is still more to be done.
Cheers