icache.co.uk was the site. Last time it ran the UK cache total was about 69,000.
The site still operates, but just for event caches. Shame the owner closed it, it was a great source of stat information. However, it could never have calculated the total cachers in the UK, only the different accounts held on the current UK database, plus maybe cacher accounts on placed caches where the owner hadn't found any in the current database. (none of which it ever did IIRC)
Would have been a start though wouldn't it.
On the above post that talks about estimating on world population, what answer do you get if you proportion it by country based on the forum topic %. I don't remember seeing a section for China or India on here, that takes quite a few numbers out of the equation.