The census figures are a bit old (2006, 2006 and 2011) but still very informative. I've got a book by James Lewis on the demography of NRMs called Sects and Stats. He says that getting access to the more detailed information in census returns can be tricky even for academics, and expensive. It should be possible to get the figures for earlier censuses and track JW education over time. But it might cost to access the information and the information might be difficult to extract.
Unfortunately the UK census returns are totally useless when it comes to JWs because it tends to lump them together with other groups under "Christian".
Census results in many countries consistently give higher numbers for JWs than the Yearbook numbers. At the same time there are always far fewer Mormons in the results than the Mormon church claims. (They claim 146,000 members in Australia for example, while the census shows 52,000, a pretty huge and fairly typical LDS discrepancy)
It's also interesting to note that the Canadian census results show stagnation and decline in JW numbers even as the Yearbook numbers continue to climb. It may be that while the official number has grown, there are fewer "cultural" JWs now than in the 1980s.