If you buy from "show" stock, then it is quite likely they will be tatood with an individual number which relates as Gavo and Farmer say to their individual number registered with the BPA and may also contain three letters. Wizard mentions the other method for some breeds which is ear notching and is still done.
This has nothing to do with legal identifcation, which will always consist of your Defra herd number (one or two letters followed by 4 numbers). Except for pigs being shown, they do not require individual ID
Therefore to answer the original question - slapmark when you take to slaughter.