Hi Oxana, I was with SWPP for a number of years but didn't end up really making use of them so I recently cancelled my membership. However, there are a number of benefits to being a member of a photography association, including training opportunities, mentoring, reduced rates for insurance, competitions and listings on their website. I think it's up to each individual person to assess how much you'll make use of these benefits. Also when I was first starting out I felt that it added professionalism and reputability to my brand, and gave clients added confidence in an emerging photographer, although now that I am more established this isn't so much of an issue. There are lots of associations, not just the SWPP, also the MPA and the Photo Guild etc. The best bet is to go to a trade show like Focus On Imaging (coming up in March) where the main ones will all have stands, and have a look at what they have to offer and have a chat with them.
http://www.focus-on-imaging.co.uk/