To be fair, the UK has too many organisations and you can't join all of them.
Personally I'm a member of LAA, BMAA and RAeS: I could of-course join AOPA, GAPAN and you could think of a few more as well and it's easy to criticise people who belong to the wrong list of associations.
Personally, I think that we should all join and engage with at-least one of the national bodies, but be patient if not everybody joins our own favourite.
G