I have set up eBay alerts and I search bikeshd every day, which pulls in all the gumtree stuff. The police did tell me about that, although they didn't say anything about bikeregister.
About 5 years ago the police gave me an Immobitag from Immobilise.co.uk which has got some sort of transponder that will react when scanned by the police, but of course they would have to find it first, and have a scanner which sounds unlikely now if they are promoting bikeregister instead.
It did have a sticker for deterence purposes which I had on the bike, but I had literally just had it resprayed so it wasn't on there anymore. In fact I doubt I would have had time to re-apply any bikeregister stuff either - next time I'll know better.
There is also some new bluetooth gadget that I heard about which will emit a small signal that anybody carrying a smartphone nearby would pick up. Apparently the smartphone app (which cyclists would have to be persuaded to download en masse) will then transmit the 'ping' to the internet where a central database will then alert you that your bike has been located. I think this is probably only in the US or Japan at the moment, I haven't seen it anywhere. Next time!