We're headed to Scotland in June, and unless I hear something to the contrary, we plan to get SIM cards from Giffgaff. They claim the O2 network is "the best", but if the UK is anything like the States, you can't believe coverage claims. 5G is not that big a deal since the only data usage will be for email when there's no wifi and occasion Google Maps for walking instructions.
I don't expect any carrier to work in the hinterlands, but I need a local number for restaurant reservations, lodging confirmations, and the like. We'll be Oban, Lewis, Orkney, Shetland, Edinburgh, Inverness, Fort William, Dunkeld and places between. If I need to call back to the States, it will be for an emergency, in which case cost doesn't really matter.