Strange and hypothetical thought: could Bill purchase a US or international model of the iPhone with US or international iCloud account capable of ADP to activate ADP? Would this allow somebody living or working in the UK the ability to use ADP?

You can just register US Apple account or switch region to US and not bother with UK regulations.

Only disadvantage that you will need to pay fot services via US gift cards as you need US bank cards otherwise.

Or an international bank account & card like you can obtain with Wise and other services.

Wise cards are issued in your country of residence. It's very tricky to get US card remotely as UK resident. Only way somewhat accessible to normal people is AMEX global transfer, but even then you need to have ITIN and US bank account.

AFAIK you cant just assign non-US bank card to a billing on US Apple account. At least it was the case before and at least for purpose of switching account region.

Thus you use USD gift cards since they easy to buy.