It's a tough one..
I like having the Enter and Shift in the "base" ISO kit only (the UK version) and not including them in the International kit. That way you do have to buy both to get say a Finnish ISO layout covered, but if the International doesn't tip you at least have the fallback option of having all the keys you need to cover your ISO board, albeit with UK legends. It's expensive for those who want the full ISO layout for their region, but those who really want it will pay. I don't see a better solution, since separate kits per region probably won't meet MOQ, and if you have Enter and Shift in both kits and someone orders just the international kit and it doesn't hit MOQ, they're stuck with an ANSI set to put on an ISO board...
Having the UK caps is good for resale, too, so most European buyers won't mind having to get the extra UK caps just to have the Enter and Shift (it's only a few dollars extra - $4 or so - and you increase the resale options).
For instance, in the Retro SA buy I ordered both the "base" ISO and International kits. Fortunately they both hit MOQ and were produced, but I'd still have been happy with having the UK legends in case it didn't. As it is, I now have a set I can resell (but I won't
I like it too much) to anyone in EU (incl. UK) or the States and it'll have the correct legends.
I actually mostly use ANSI layout now, though, and a fair number of other EU keyboard enthusiasts do the same if they're interested in aftermarket caps since it gives so many more options and it's rare that International kits hit their MOQ and many sets don't even have the International kit (like Danger Zone, for instance).
TLDR: Having a "base" ISO kit in UK legends and a bigger "international" kit with the special legends per region, but no Enter or Shift is the best option, IMHO.Either that or separate kits like in Sci-Fi, but it's a big risk. For those who order a kit that doesn't hit MOQ they'd have an ISO Enter and Shift (since they're their own kit), but would have 2 empty spots on their board and wrong legends on others. If you could somehow guarantee MOQ on all language packs then that would be nice from a buyer's perspective, but a nightmare from a GB organiser's perspective. Of course this could also be handled by a custom GB system where people can choose fallback options, such as: "If this kit doesn't tip, would you like the UK kit added to your order?", but you'd have to run the whole GB yourself then (traditional-style, with sorting, packing and mailing the sets yourself), as I don't think Massdrop, PMK or any other GB system has an option like that.