Kenilworth makes a good point here. Happily, I don't think we have quite the same problem of under-reporting in the UK but there will always be some inertia between discoveries being made and proper descriptions being published. So any data produced would never be quite up to date.
One way of obtaining useful historical data for the UK might be to go through all the main guidebooks, which quote the years of most major extensions. This would allow a reasonable estimate of the overall amount of new stuff found each year over the last Century or so. (It would be interesting to see any obvious hiatus during the two great wars, for example, or due to foot & mouth disease outbreaks, or the 1967 Mossdale Caverns accident, etc.)
However, this would tell you more about breakthroughs, rather than exploratory activity - an important distinction.
A number of years ago, when BCA officers were sorting out our insurance scheme, there was a need to know the total length of cave passage in each major caving area. (Why insurers ask for such information leaves me puzzled but there you go.) A volunteer was sought for each area to compile these data, allowing a ball park figure to be arrived at for the whole UK. Such an exercise might be done to try and provide the information Rob asks for? (I'd normally volunteer to do this in the Dales, as I did for the insurance query, but I've too many things on at the moment to offer.)
If someone was willing to co-ordinate this (Rob?) it could be done . . . .