I'm not a great fan of gyms. But when I learned I was going to be spending 6 weeks in London I thought what the hell? Virgin Active have a great 'pay-as-you-go' monthly membership, which was perfect for me.
Having not been in a gym since 2005 I was amazed to discover the new addition, the power plate. At Virgin Active Wandsworth they have lunch time power plate classes that last about half an hour, which is ideal if you only have an hour's break!
I also tried the aqua aerobics class which was fun and dynamic.
At the end of a workout you have the sauna and steam rooms and the jacuzzi to relax in.
Though it is not the biggest gym in the world it is perfectly adequate and you've got everything you need.