ChristianObserver,
If only life was that simple...
So far as the UK goes, such information in the UK would be covered under the Data Protection Act 1984 and as such may well be considered 'priviledged information', this is akin to the communications between doctor & patient, priest & parisher (e.g. confessionals) and lawyer & client. In order to seize this information a court order would need to be obtained from a Circuit Judge following an 'inter-party' hearing.. or the holder of the info (e.g. the WTB&TS) may volunteer it.
I would imagine that the latter is unlikely to happen... the former is only likely to happen as part of a criminal investigation, and then the holder of the info (e.g. the WTB&TS) could only for forced to provide such info. as is pertinent to that specific investigation.
The justice system is geared as such that the police cannot go on 'fishing expeditions' hoping to uncover something that may or may not exist, and that is a necessary protection for all, not just the JWs.
Somewhat more cynically, the criminal justice system may well consider the existence of said 'register' to be of little to no objective consequence and maybe not even worth the hassle, in spite of our own individual and more subjective concerns and curiosity.
Bottom line, I don't know ;)
FreeToBeMe