Some friends of mine and myself were intrepid enough to try out the Third Space and we were quite pleasantly surprised by this lively fitness centre. It has a wide range of exercise equipment and classes on all 4 levels. They have equipment like a sky lit climbing wall(instruction is available),boxing ring, ping-pong table,cybex,life-fitness and Stair-Master machines. If you are super fit try out the hypoxic chamber where the individual can exercise at reduced oxygen levels to an altitude of 8,500 feet.
If that is not enough incentive to justify the hefty joining fees then perhaps the temptation of eating at the Fresh&Wild juice bar, use of their laundry service(35.00 for 10 washes),wifi access,Redken shampoo and conditioner and a live DJ 4 times a week might be worth it.

One of the nicest, if not the nicest gym in London. Great facililties - weights, cardio, pool, sauna, health clinic, massage, climbing wall, etc. Very central location. Friendly & polite fellow gym-goers.