REALITY CHECK
I have been combining, or trying to, commercial flying with Instructing for nearly 25 years. It is not easy. I don't know where you live, probably down near Heathrow since you say you are BA (couldn't resist it could you Nigel), but you will find that there is enormous resistance to airline pilots taking instructor posts at flying clubs, even if they are only part time. You can argue that it is your hobby, it's fun and you just want to give something back, a lot of clubs simply won't buy it. Other instructors simply see you as a threat to their jobs, "taking our hours, etc." You may feel that you are giving the club the benefit of your experience, but believe it or not, lots of club members cringe at tales of flying B777s to sun-kissed beaches. If you break even on your investment, good luck.