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.  

  Comment signaler mon vélo comme volé?

Hoe registreer ik mijn fiets als gestolen?

Wie melde ich mein Rad als gestohlen?

  Come posso registrare la mia bici come rubata?

 ¿Cómo registro mi bicicleta como robada?