Immobilise is the official stolen property database, and is free to use, but only the police have full access to it. I’d still put my bike on it though. Apparently extra-cheeky bike thieves have been known to register stolen bikes as theirs using it, so it makes sense to beat them to it.
Bikeshepherd seem to be gaining a bit of a foothold, they’ve recently become an official partner of Cyclescheme which gives them access to a lot of bikes.
http://www.cyclescheme.co.uk/community/featured/protect-your-bike-with-bike-shepherd
Their smartphone app is free and means anyone (with a smartphone) can check who a bike belongs to, but I’m still not 100% convinced by their metal sticker tags – surely any savvy thief can put another sticker over it or buff off the code.
Datatag claim to have supplied all the police forces in the UK with the necessary scanning equipment to tell whose a bike is, so there should be no excuse for not getting one back to you. I’m not sure whether most officers would think to check though, given how few bikes are recovered in the first place, and what tiny proportion of those must be Datatagged.
When it comes to stolen bikes being recovered, we’re talking just a few percent of the number going missing, and the tagged ones are a tiny fraction of that. With the numbers so small overall there’s no definite winner and I’d probably go with as many of the above as I felt I could afford, as they all increase (however slightly) the chance of your stolen bike being recovered.