You have to make it worth it, by going. My gym membership pays for itself if I go at least twice a week, which I do. It's a council run gym so cheaper than a lot of private ones. It's hard to say if its worth it for you as depends on your use. Work out what a session would cost and see what you would have to do to make it worthwhile.
I don't know about in London, but my gym pays for itself over and over as long as i go 3 times a week, but it's absolutely nothing like London prices. When I can't make it that often it's usually health related and my gym are great and suspend my payments when I can't go for a month or more.