Hi - I have just gone through this so may be able to help!
Check with your slaughterhouse as to how they require your pigs to be marked. I thought I could take mine with a slap mark but was told that this can rub off during processing - I had to tag mine, which although sounds a little bit off putting was absolutely straight forward. It can take a few days for tags to come through (you have to have them made up with your Defra herd/flock number).
Our local slaughterhouse was very helpful and talked me through all the regs and paperwork that I needed.
Good Luck!
Myrt