You really have a professional responsibility under the ANO to notify them immediately as soon as you were diagnosed, even if you voluntarily stopped flying yourself. They'll send you a letter informing you that you are "temporarily unfit", and include a list of information they'll need from your oncologist when the treatment is over. Upon receipt of that they'll review your case on its own individual merits and determine whether or when you can regain your medical previledges and what restrictions (temporary or otherwise) you will have.
I'm going through this myself. My chemo (for testicular cancer) ended in Jan and I'm waiting at the moment for my oncologist to write his report to the CAA.
I have no idea what the ramifications might be if you didn't notify them of your illness and treatment, but I would assume at some point in the future it's going to catch up with you. Speak to the CAA at Gatwick, they're really quite sympathetic and will do whatever they can to get you flying again. Don't worry about not having informed them until now. Events overtook me after I was diagnosed and I didn;t get around to it for a couple of months into the treatment and they were fine about it. (I stopped flying immediately I got the diagnosis though. Not that I had any choice, what with an operation within days and embarking on chemo only days after that.)
Si