Just picking up on a few comments and queries ...
We use Shearwell tags which are big and ugly but easy to read and really quite good at not coming out, and Shearwell do offer a 'free replacement for life' option (for which you forgo a few percent off - ie, for which you pay, but it's not very much when you have a lot of sheep.) I doubt if I would use Shearwell for small numbers of small-eared pedigree sheep.
At time of posting, Shearwell charge 64p for a slaughter tag or 79p for a 'set' - ie, yellow electronic plus non-electronic match tag (your choice of colour.) We buy all paired and sometimes use the yellow (electronic) tags as slaughter tags (ie, on their own) for lambs going to slaughter, which is strictly not correct but no-one has minded yet. The spare non-electronic match tags then get used re-tagging older ewes who've lost their old (non-Shearwell) tags - again, not strictly correct but no-one has minded yet.
commercial farmers get a reduction in carcass price if it is not EID tagged.
I think it must depend on the slaughterhouse / buyer, SFS. Our buyers don't care a fig, which includes Welsh Country Foods, who supply Asda and Tesco. (No they don't label our lamb Welsh, be calm my Welsh friends. They ask about locale on the form, ours is North Country.)
I have threatened before to share with you all what I really think about EID but I need to get on and feed my sheep so won't start now. But I will do later on.