Legally in both Scotland and England and Wales, there is no requirement for individual ID for pigs unless being shown or exported.
The requirement for slaughter is either "an ear tag with the letters "UK" followed by the herdmark of the holding from which it is being moved" or "a tattoo showing that herdmark (with or without the letters "UK")" a tattoo can be a slapmark, see further down.
Additionally in Scotland only "a slapmark showing a unique individual identifier being the herdmark which identifies the holding of departure; or an alpha numeric identifier allocated to the keeper by one of the marketing or processing groups" There are currently three groups; namely, Vion (processing group), Scottish Pig Producers and Scotlean (marketing groups).
A ear tag must be..."(e)sufficiently heat-resistant that neither the ear tag nor the information printed or stamped on it can be damaged by the processing of the carcase following slaughter"
A tattoo "must be applied either by tattoo forceps, in which case it must be on an ear, or by slap-marking equipment, in which case it must be on each shoulder and must, in either case, be easy to read."
Many/Most abattoirs won't accept tatoo's in the ear, some don't accept ear tags, all as far as I know all accept slapmarks, but check.