The Dogs Trust currently have a nice freebie for dog owners. It costs around £20-£30 to get your dog microchipped at the vet. However, you can currently have it done for free at one of their Rehoming Centres. There is a link at the bottom of their page about Microchipping that takes you to search for your nearest centre. You then just contact the centre directly to book a mutually convenient time. Once your dog has been microchipped, please consider making a donation, however small, as the Dogs Trust is a charity.