ID is only required for transport between holdings.
A pig that is born on a holding, lives on the holding and then dies on the holding never needs ID
A pig under one year old (NOT going to slaughter) going to another holding can move on a temporary mark, which only needs to last for the duration of the journey - so if you buy say an 8 week old weaner on a temporary mark, and it stays on your farm, again it needs no ID.
A pig over one year old (NOT going to slaughter) going to another holding needs either an eartag or a tattoo (typically ear) - legally it can have a slapmark, but you don't want to do that as it causes problems for ID later - so if you buy say a breeding sow, then it will come with a tag/ear tattoo of the holding it came from, and needs no further ID whilst it stays with you. IF the tag falls out, it does not need to be replaced.
If you sell a pig over one year old to another holding, you will need to put a tag or tattoo on it with your holding as per the above.
A pig going to slaughter AT ANY AGE must have either an eartag or a slapmark (both shoulders). Legally it can have an ear tattoo, but the abattoir will probably refuse it, and some abattoirs only allow slapmarks
all comes under The Pigs (Records, Identification and Movement) Order 2011 (similar laws apply to Scotland, Wales and NI)
http://www.legislation.gov.uk/uksi/2011/2154/contents/made