I can only say I would buy it for up to 120 GBP per active account (where active account is not active in 2014 but in Aug 2014).
If the active account number is roughly the same as in July, then my price would be less than 1 year income. What is the value is really hard to say. Internet projects/parking domains etc. are sold in range 0.5 to 5 yr revenue most of the times. Selling an affiliate account is definitely in the lower range because the question here is what is fishy with the account. I do not care what the seller will tell me, because I know it will be a bullshit. Nobody is selling golden goose. This is why I would be very conservative here.
The value of the affiliate account is determined by asymetric information that is known only to the affiliate account holder. Therefore the market price will be always well below.
If you talk to God, you are praying; If God talks to you, you have schizophrenia.