I'm in the United States so any input to my British friends might not be applicable there, but over here, if money isn't an issue, you can continue on after retirement as a CFI with little trouble at all. I assume you have our equivilant of a multi/ATP. If so, I would assume just taking the written might be all that's required at your end of the pond.
Don't know what the GA market is now in the UK, but over here it's pretty flat. There are jobs for part time CFI's here but a full time job instructing at any kind of livable salary is a chore to find.
As for age, no sweat. I've been in flight instruction and associated with training for over fifty years and it's always been my experience that when given the choice, new students will choose experience over youth all day long.
Just learn to speak their language. Personally I drew the line at listening to Guns and Roses :-)
Dudley Henriques