In the UK our associations are fragmented, and unlike US AOPA which has a very broad parish, there are various "clubs". Most are worth joining, but get the right one. I'd suggest thinking along the following lines (in no particular order): -
Light permit aircraft, homebuilding, private flying in more interesting light aircraft - PFA
Anything to do with microlights - BMAA
Anything to do with gliders and motorgliders - BGA
Ballooning - BBAC
Instructional community, operators of "conventional" (e.g. Piper and Cessna) light aircraft - AOPA
Non-flying enthusiasts - the air league
Professionals in any area except PPL instruction - RAeS
Competitive pilots (except gliders), pilots who want to be in the guinness book of records - RAeC
Non flying enthusiasts - the Air League
Deep interest in navigation, avionics, charts, etc. RIN
Hope this helps. Personally I'm a member of PFA, BMAA and RAeS and have no regrets about any of them.
G
N.B. The old "AOPA got us JAR-FCL for PPLs" thing has done the rounds for a while. I suspect there's some truth in it, but most of the guilty parties have moved on to other things by now anyway, so there's little point in banging on about it now.