Leri,
Suspect that two ID's are in effect here, the BPA no. and the breeders herd no.
Notching on saddleback is the pedigree ID (equiv is a tatoo on GOS)- the alternative BPA ID is two tags, but these would only be needed if the breeder elected not to notch. So if she is notched, then the tag would probaly not be related to BPA ID, but rather with a herd no. so that she was legal under Defra to be moved over a year old.
Therefore check you BPA paperwork and BPA website on notches. If she has notches that match the BPA paperwork, then you should be fine. Next check the AML2 that came with her. This will probaly say metal ear tag herd no. AB1234 which would confirm that she moved with a tag unrelated to the BPA pedigree. As Lillian says, pedigree BPA tags are usualy (though not always) plastic, not metal. Metal are typically used for slaughter or herd. no. Finally you should be able to confirm this with the person you bought her from.
From what you say, I would take an evens bet that the tag was just herd. no.
As to your main point - yes the lavbelling of pigs for managemnt, BPA and legality creates a minefield which i still struggle to explain in simple terms. It is very cionfusing !