I only belong to the US AOPA. I fly a lot in the US, and they have great online resources and do excellent work in defending GA there.
I think that advocacy in the UK is weakened by having so many splinter groups; of course, the EU regulators have already damaged EU GA too much for a "GA serves America" style campaign to be run over here. The European attitude seems to be "how can we bring these rich playboys down to our level", rather than the US view of "wow, how can I get up there?"