The important thing is that the chip happens first. If it was put in years ago, as long as it gets scanned before your vet begins any of the PETS procedure stuff, it should be fine. Just make sure that the chip type is OK with DEFRA. Their website says:
We do not specify a particular type or brand of microchip to be used but, in Europe, ISO (International Standards Organisation) Standard microchips meeting specifications 11784 or Annex A of ISO Standard 11785 are generally used.
If the microchip does not meet either of these ISO Standards, it will be your responsibility to ensure that the microchip can be read upon entry to the UK. This may mean that you will need to provide a microchip reader (at your expense) to allow the chip to be read. Some ports of entry may have equipment capable of reading other types of microchip and you should check this with them before you travel.
If the officials at the airport in the UK can't read your chip, they won't let your pet in. (Though you can buy a scanner and bring it yourself just to be extra safe.)
My dog came pre-chipped, but the format was one of the ISO ones, so it was OK. I just put down the year that I got him as the microchip date.
The important thing is for the vet to verify the chip number/location
before each procedure. So before you get the rabies vaccine done (if needed), before the blood is drawn for the titre, before the flea/tick medication, etc. At least one poster here had their animal(s) put into quarantine due to a chip going missing. (I also had to replace my cat's chip as it disappeared at some point over the last 3 years.)
