Have a look at the new website
BALPA - Home
It should answer your questions.
A union gives its members a co-ordinated voice when it comes to negotiations on Pay/Conditions etc.
If you're not in the union, you don't have a say.
If you have an incident, the legal cover will help you. If you're not a member, the cost of legal fees could be very high, and remember you need to have been a member for 3 months before legal will assist!
Nearly all UK airlines have a lot of BALPA members. Some with membership numbers as high as 95%.
They have information on tax issues.
The subscription is heavily discounted first year then a further discount year 2.
Oh, and the subscriptions are eligible for tax relief at 66% I believe it is now.
Hope that helps.