The statistics you're after are more or less available
here. I say "more or less" because the information seems to be rather dated (in Canada, such statistics are updated on a quarterly basis [see
here], but that doesn't seem to be the case in the UK).
It looks like Whirlybird and ozplane were essentially correct about the number of PPL(A)s. Frankly I am surprised that the number is so low; you seem to have roughly the same number as us, yet your total population is twice the size of ours (~61 million vs. ~32 million).
After all a UK CAA PPL is issued for life, even if you decide to give up
I don't know for sure, but I suspect that the CAA doesn't consider a pilot without a current medical qualification to hold a "valid" license.