Another benefit which hasn't been mentioned is that as a BALPA member you are part of IFALPA thats the INTERNATIONAL Federation of Airline Pilots Assoc.
Our job is international so if you have an incident in, shall we say, a less friendly environment you have some protection.
Many years ago when Swissair ran off the end of the runway in Athens the pilots were immediately placed in gaol by the authorities. It was IFALPA (not the employer) that lobbied to get them speedily released pending a full and proper investigation.
For my money this is well worth the small membership fee.
BALPA also have many technical and safety committees which lobby UK Government for change etc.