I have been paying £50 / month for my regular Fitness First membership. This was for the top tier clubs, full access in downtown London, on and off peak allowed.
I recently looked into joined BUPA personal health insurance, specifically for their Fitness First deal. As I'm young and fairly healthy, Bupa quoted me £16 a month for their insurance, and I received a free 12mth, unrestricted Fitness First membership.
Needs to be noted though... if you try the same please make sure you read the fine print and call your local club to double check they accept the Bupa voucher and will not try to restrict your club access to off-peak, or something else. I called the club, read all the fine print on the website, and grilled the Bupa salesman extensively to make sure the deal was legit. Also would recommend signing up towards the middle of the month, as it takes a couple of weeks to get the paperwork sorted, and the salesmen's deadlines for commision are usually the 15th or 20th.. so they will be more likely to help you out in order to make their sales.
Summary: £50 per month drops to £16 per month for a full year's contract.