Many London gyms offer free guest passes to potential new members to tempt you to join. Guest passes are a great way to test out a gym, you should also negotiate to receive guest passes when you sign a new gym contract.
Kinetika Gyms are currently offering a FREE 5 DAY membership pass at any one of their branches! Putney, Balham, Battersea, Sutton, Tooting or Wandsworth.