I think another good reason for being a BALPA member is that they are a part of IFALPA - our job is international and we share common challenges with our international comrades.
I believe BALPA has done much to help improve things over the years - they are not perfect but who is?
I recall that many years ago when Swissair went off the end of the runway at Athens and the crew ended up in a Greek jail it was IFALPA that secured their release. Surely a good reason to be a member and well worth the money.