"Does anyone else in the whole world believe that the people of mainland Britain should be allowed to vote for the next government of NI?
Has Jim completely lost it this time?"

There could be a twisted method in the madness! There have been polls showing that far more Britons (who show any preference) favour a united Ireland over Northern Ireland being part of the UK - whereas there was a recent poll in Northern Ireland showing support for a UI at only around 36% or so. Hence if the British voted in a referendum they could well vote for a united Ireland whereas the people of Northern Ireland probably wouldn't. Though of course that would amount to kicking the people of Northern Ireland out of the UK against their will and I imagine when it is put that way there would be little support for that amongst the British!

Of course you are right only people living in Scotland voted for Scottish devolution etc etc. So the people of Northern Ireland should have the right to state whether they wish to remain within the union or not and not have their voices swamped by the far more numerous English and Scots most of whom don't have much of a clue about NI and many of whom don't care about NI. The British accept that principle, and 90% and more of those in the Republic accepted that as did an estimated 90% plus of Catholics in the north of Ireland.