I used to be with PruHealth, before the Vitality programme started. I would be very careful that you are not paying a premium for their commercial tie ups. I moved to AXA three years ago and saved over £1,000 pa for my wife and self. Weirdly I'm going through the same cost comparison exercise at the moment and my broker found another AXA product called Health On Line which is essentially the same with some minor differences and saves several hundred on this year's premium.
Worth checking the competition and then seeing if the value of the gym offer is worth any extra.