As JulieS says they need to be metal. This is because as part of the process the hair is lossened by putting the carcass in a scalding tank. Plastic tags melt so are not acceptable. There are plastic tags that survive the process, but don't buy them. An abattoir cannot know from just looking that they are scald proof,so will reject.
Ditto as JulieS says only tag the day before, and take the pliers and spare tags with you. That way if one has come out on the journey you can replace.
For abattoirs in cornwall, we had one recommendation Penryn R J Trevarthen
The reviewer said:
"9/10 (I'd give 10 but I think their sausage making is a bit expensive!) Its a family run concern that has expanded in the last couple of years given the popularity of private kills Pigs taken on Monday and Wednesdays
Everyone was very helpful - no hullaballoo, and we weren't hurried at any point.
They have a full butchery service - the butcher operates on site and you can ask for any variation of cuts you want - the butcher Peter was very helpful for us as a couple of Newbies. They will also cure meat"