So basically, paper-shuffling scribes involved making agreements and payments between themselves. Much the same process as you go through when you get your insurers to agree treatment under your travel policy.
I find it incredible that the UK, which pursues equality and anti-discrimination almost to the point of mania should countenance such a situation. There would be howls of protest if it was suggested that somebody living in the UK but originating from outside received differential medical treatment or the provision of such treatment was possibly jeopardised and was dependent on the basis of where they came from. Imagine this: a person born in Nigeria but legally settled here is told "We've got to run the details of your treatment, which we'd happily provide to a person born in the UK, past NHS Lagos and see if they will pay for it before we go any further". I can see that going down well. Yet here we see a government sponsored lunatic scheme which does just that to a UK citizen.
The politicians who championed devolution (long since disappeared into the sunset, natch) should hang their heads in shame for having devised a scheme that does nothing but promote division and rancour.