The girls would need your herdmark to identify them if they did go to slaughter. Usually breeding tags are plastic ones, and you would use a metal one for the abattoir.
I get my tags from Mole Valley Farmers in this area (Devon). Depending on what type of tags you have you buy an applicator with the first lot. Mine came with directions of how to use.
The BPA send out lots of info about tagging and identification.