I'm with
@consultant on this.
I have worked in offices with gym, dry cleaning, subsidised restaurant, games room, etc provided for free to employees and contractors. No BIK issues for either.
If the company is providing office space, and invoicing for office space/hotdesking, and there is complimentary use of a gym, HMRC should have no issue with the cost of it deducted for CT purposes.
Regus offer this at some of their facilities.
However, in the OP's case, if the company is primarily providing gym facilities, and invoicing for gym membership, HMRC would not unreasonably object to the OP saying that the cost is for office space.