Somewhere in the depths of the CAA website is data on annual license issue numbers. It is around 3k a year for the PPL.
Nobody knows how many are valid, let alone how many actually fly. However it is well known, and the CAA have occassionally said so, that some 75% of new PPLs expire with less than 10 hours flown after getting the PPL, and about 90% stop flying before their first renewal (i.e. within 2 years). So the attrition rate is pretty high.
Somebody who is clever with stats could work something out based on this.
My guess is that about 10k pilots are hanging in there, but whatever the number, their annual flying hours must cover a wide spectrum.