nrl1965, firstly congrats on beating cancer. That is one major hurdle you've overcome.
Ok, the ATPL thing. You're never too old etc. However, you may be too old to get a reasonable return on the investment.
The medical. The UK CAA are pretty harsh about cancer. I have a couple of mates who are going through the waiting game right now after losing their Class 1 because of cancer. So, good luck, but don't be surprised if they say no.
What you and only you can work out is can you afford the cost of training without seriously impacting you and your family's financial future. If they are ok with you spending (admittedly your money) on a CPL & IR and thus possibly missing out on holidays, uni whatever may come up then it's ok.
But you really do have to sit down and objectively go through the costs and what you'll need to give up.
Nights out & holidays & new cars become "hours".
Your dream is (now) your family's dream. Make sure they're in on it.
That's just the cost of training. You'll be about 48 or so by the time you've finished, possibly older.
If your goal is to be a captain on a 744, sorry, you are too late.
If your goal is to be a captain on a short haul 737 or A320, then you might just make it but more than likely, not.
If you want to instruct then age isn't an issue, may be a help (especially your background) but bare in mind you won't make a lot of money.
If your plan is to keep the "day job" and instruct on the weekends, I can see no problems other than lost time at home. (it could be a good thing
If you want the CPL/IR as a reward for beating cancer then that's ok too. Why not? Although, to be honest, I'd rather see you go and have a huge flying holiday renting a bugsmasher and flying around Australia (a bit flooded at the mo) or Canada. That'll be worth something. You'll do something far fewer people have done.
Before all the apologists come on and bleat about "it's not about the money", try buying a trolley load of groceries with your log book. It won't work.
My log book(s) has over 7,000 hours in and Tesco's just laughed.
Sainsbury's took one look at my battered green book and threatened me with security.
The harsh economic truth is that you have a limited time to make up the money you'll have spent plus save for retirement.
I wish you the very best of luck.