With compulsory microchipping for our dogs in England starting from 6 April 2016 (I'm not sure if Scotland and Wales start on this day). It is worth pausing for thought that when putting foreign bodies into our dogs, for some, it has not been plain sailing. www.microchip-implants.co.uk/index.php/adverse-reaction-case-studieswww.chipmenot.org.uk/
For anybody not up to date with microchip adverse reactions, any microchip that has moved from it's original site of implant must be reported by law to the Secretary of State through the Veterinary Medicines Directorate. Your vet should be happy to do this. The microchip on one of my own dogs moved down to his upper leg and my vet has reported it.
The Microchipping of Dogs (England) Regulations 2015
10.—(1) Anyone who identifies an adverse reaction to a microchip or the failure of a microchip must report that reaction or failure to the Secretary of State.
(2) In this regulation “adverse reaction” means—
(a) any unnecessary pain or suffering, or any pathology on the part of a dog which is caused, or appears to be caused, by the implanting of a microchip; or
(b) the migration of a microchip from the site of implanting.
(3) In this regulation “failure of a microchip” means failure to transmit the number encoded in the microchip when scanned by an appropriate transceiver