The latest figures I can find
here on the CAA site are from 2004-2005 during which time 308 IMC ratings were issued, compared with 1913 PPL(A) licences. Of course, many of the IMC issues would have been to people who gained their PPLs in earlier years, so those figures on their own don't tell you the overall percentage of pilots who get the IMCR.
You could add up some numbers from
here as well.
The index page to the statistics is
here.