“they believe the UK of GB&NI to be so wonderful”
Possibly.
Certainly they believe they need to carry out the “will of the British People” - or at least the ones they think they understand/agree with.
I think it’s clear that some people voted Leave with the understanding of some level of isolation and the hope that somehow it might make things ‘better’. Stopping immigration was only one element.
Other Leavers of course believe that ‘free from the red tape shackles of the EU’ BRITAIN will be able to dominate/ravage the rest of the world (only in a free trade sense of course...).
Some of these FreeTraders, no doubt, excepted the EU to collapse allowing more opportunities/less organised competition.
There is the extra narrative (from all sides) that ‘people didn’t vote to be poorer’.
Apart from being an unknowable, the chances are that most people didn’t think about it, or assumed that the side of the bus promise and ‘no more of our money sent to Brussels’ ‘fact’ meant everyone would be better off.
A small percentage would even be ok about being worse off if it made some things got better - whatever they were hoping for.