Virgin active lenasia unfair business practice with regards to contract cancellation

I opened a 24 month contract for my wife last year august with Virgin active Lenasia. We have now moved away from Lenasia and my is pregnant so I want to cancel the contract. I have read the contract and no where does it state a set percentage has to be paid. The actual wording is " If you pay your monthly membership fees by debit order and you wish to cancel your membership before the expiry of your Commitment Period, then you must give us 20 business days’ written notice of termination and pay a reasonable cancellation fee plus any arrears".

However I am being told that I have to pay 40% of the remaining contract value. How is this a reasonable fee ? I expect Virgin active to justify to me how they have come to a percentage of 40% before I pay it. And if this has been the case for many years why does virgin active not state 40% in the contract. This is not good business practice at all!