You can start a gym membership whilst in an IVA, but I doubt that the creditors will allow the fees to be added as an allowable expense. If you can fund the membership from your already agreed expenses there should be no problem.
If you haven't yet started / agreed expenses, it could be covered by an "entertainment" allowance ( I was allowed £25 back in 2010 --- but that was for three of us and didn't go far!) or absorbed by a quid added here and there in other allowances.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk
Agree - if you can fund it from other allowances you should have no problem - hopefully your gym won't do a credit check.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley. http://kallis3.blogs.iva.co.uk