Microchips are the best way to help reunite keepers with their missing pets. The responsibility is with the keeper to make sure that their pets are registered on a DEFRA-compliant database with up to date details. This is the main method of reunification if a pet is to go missing.
Dogs
Legislation states that dogs must be microchipped and registered by the time they are 8 weeks old, with details kept up-to-date. If not, keepers run the risk of a fine of up to £500.
Cats
Legislation states that cats (England only) must be microchipped and registered by the time they are 20 weeks old, including cats that usually only stay indoors. Again, failure to ensure this runs the risk of an up to £500 fine.
The rest of the UK does not have the same laws currently.
Further information on the microchipping legislation can be found here.