Neither Abattoir or State Veterinary Service or Meat Hygiene Service will care unless or until there is an issue. Eg your pigs show some disease in meat, a medicine trace check fails, or someone has an non-id or wrong id’d pigs and SVS or MHS start cross checking to reconcile whats in the pens with the paperwork. Or someone else has a pig with similar no. and causes confusion (eg if you use breeders tags and breeder or other customer he sells to uses same abattoir as you and you both have pigs in that day). Even then it is unlikely, but it is possible, hence you run a small risk.
More likely this may be picked up is on an SPS check, or a Trading Standards or AHVLS Check of your movement book, all of which if done properly (and most are) will pick it up and may (or may not) action – may well be classed as an intentional SPS breach, so fine and reduced SPS for that year, or fine under PRIMO 2011 – or of course both.
There is of course a high likelihood that at some stage in the future Defra will run a check against the E-aml database to see who's using the wrong numbers, and may (or may not!) take retrospective action – Defra always seem keen to be displaying to the EU that they are finding wrongdoers !
By the by - if you do ask breeder to tag and then simply let these be used when you go to the abattoir, you do increase the risk that you'll not get your own pigs back - for the simple reason if other people who buy from that breeder also do this, then the rest of the same litter will be ready about the same time, and with many abattoirs only doing pigs once a week, it is entirely possible that several lots arrive on the same day with the same number - and given four pigs on the hook with the same tag no.s and two customers sending two pigs each, the chances of mix up are increased. Maybe unlikely, but just a thought!