Thank you for your replies, everybody - they are all very helpful. I understand what people are saying with regard to the difference between AGSOs and FISOs - to be honest, I'd be grateful to do anything, so it's all good fun for me and I don't mind what I'm doing.
Alas, I will never be an ATCO despite how much experience I get, or how much hard work and determination I put in. Unfortunately I am colour deficiant hence I wouldn't be able to get the medical. I don't know whether colour vision is essential for all ATCOs and something which is regularly "utilised". Whether it is or it isn't, I can't see the CAA taking pity and changing the rules.