A Stolen bike must be reported to the local police in the first in place.

There are a few websites in the UK that can help either with registering your bike and/or publicising a theft:

https://www.bikeregister.com

https://stolen-bikes.co.uk/

http://www.bikeshepherd.org/

http://www.stolenride.co.uk/ (London based)

http://www.bikeshd.co.uk/

If the bike has been stolen in a different country, register the bike on similar websites in that country.