Since the various replies so far are confusing me, let me try and explain the actual situation.
Every pig keeper must firstly register their holding with the RPA and obtain a CPH number. That means that Defra knows that there is potentially livestock at this address. (this is for England; check with the equivalents in the devolved authorities for local rules). When you get your pigs you must also register with the Animal Health Office to obtain your herd no. It is this number that must appear on a tag every time a pig moves off your premises. You also need to notify your local Trading Standards Office when you get your pigs so that they can issue you with movement licence forms and perform spot checks on your record keeping.
Providing both parents are pure bred and registered pedigree, your pigs can (should) be birth notified and permanently ear marked, preferably by notching or tattooing (according to the breed) by 8 weeks of age or before mixing with other pigs, whichever is the sooner. (They can be double tagged also in certain circumstances but some breeders now refuse to buy pedigree stock so marked as it is easy to abuse the tagging). Birth notification is the first step in pedigree registration. You may think that for meat pigs you don't need to bother but for all the BPA breeds (most of all breeds except Kune Kune and British Lops) you can get a Meat Certificate FOC which proves that the pigs are the breed you say they are. This is important as Trading Stds are tightening up on what people claim and this is your best defence. By the same token, although the 2 parents may be pure bred, if they are not registered pedigree, it is illegal to sell them by the breed name.
The final part of registration can take place at any subsequent time and only the breeder can register stock. Be aware that if you buy pigs for fattening and then decide to keep one or more for breeding, the breeder can legitimately refuse to register the pig(s) if they had breed faults.