It looks like you need to take a multi-faceted approach:
- Get the CAA to ask FEs to send a form back when doing IMC tests (DFC's suggestion). Recorded details could include CAA ref number and date, plus approaches and IMC hours flown since last test (I think for most people you'll find a very small number of approaches).
- Contact examiners and ask for their historical test data. If you were prepared to wait you could even get this data at the 3-yearly FE renewals.
- Instigate a pilot survey (although I'm dubious that this will yield data that could be regarded as authoritative)
You then need to try and capture all the times when someone had the ability to be VMC on top between holes when otherwise they would have been scud-running, when they were prepared to fly an approach but didn't needed to, and all the UK CPLs.
Personally I would get as much hard data as you can (first 2 bullets) and then use the rest just as unmeasurable safety benefits.
HTH Good luck!
HFD