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Since 1992, the law here in the United Kingdom states that any pet dog out in public (or able to get out into public) must wear a collar with a tag attached stating the owners name (at least surname), house number, and postcode, with a telephone number also recommended but not required.
This order stands even if your dog is microchipped. Further, since 2016, all dogs in the UK must be microchipped if they are over 8 weeks old.
If your dog is not wearing an identification tag with these components out in public, your dog can be seized and treated as a stray, and you the owner can face up to a £5,000 (yes you read that right) fine, and possibly even jail time.
Because of this, I always recommend you have on the back of your tag:
1. Your surname
2. Your house number
3. Your postcode
4. Your main phone number
Of course as stated above, number four is not required, but is very helpful to get your dog home quicker if your dog gets out.
So if you do live in the UK, do not forget to select when asked "What info would you like on the back of your tag?" the option "Phone Number + Address" to be fully covered in case of the worst.
Still got some questions about what you need and what the law states? Feel free to send me a message, I'm happy to help as much as I can!