I've just joined as a member this year, so I'm still figuring out the benefits of it. You get a regualr magazine and you get to go to the meetings and parties and the conference of course. You're near London, so you could probably get to those better than I can. There's a members only Yahoo group you can join. There are local chapters, and I've been to two meetings of mine now and that's lots of fun, to talk to real life people about writing. My group has a real range of people from people still working towards publication - NWS members, people like me who are still early in their writing career and people who have been publishing for years. All ladies in my group, though I'm told there are some men somewhere.
It's £50 to be a member for 2012 (and membership runs Jan to Dec, not 12 months after joining, which is a tad annoying) so if you're going to join, the sooner the better!
It's kind a chilled and British in comparison to the RWA and everything is all on a much smaller scale. All kinds of writers are welcome. They happily iclude same sex romances and erotic romances on their list of members books releasing this coming month that they publish on their blog.
I think the key reason to join is if there's a local chapter, because us writers are so often rather isolated with only online folks to talk to about the writing. It's nice to get together with some real poeple who don't think we're insane when we talk about the imaginary poeple in our heads.