As somebody else pointed out above; “British exceptionalism”.
I was living in Britain at the time of the Leave vote and the vast majority of the Leave “arguments” were anchored in some way or other on the idea that Britons and Britain would get special treatment because of being who they are (remember the whole “German car manufacturers will make sure we have a good deal to keep access to the UK market” argument?).
Also I’m from a very touristic country and it’s pretty easy to notice the feeling of superiority of lots of Britons when they come here, especially old people.
(The funny bit its that I ended up in England after years in The Netherlands, so ended up with Northern European standards and hence was less than impressed by Britain by the time I got there in 2006)
Anyways, all this to say that it’s absolutelly believable that lots, maybe even most, of those older people who lived abroad after retiring (so never really integrated in the local society, as people actually working there would) did genuinelly believe the idea that Britons would get special treatment. I mean, here in Portugal they’re not especially well integrated but especially the stories from the Costas in Spain is of pretty much self-contained Little-England communities, english-speaking and with their own stores and everything, all filled with British retirees most of whom never go into the trouble of learning even the most basic Spanish.
PS: Just to add that some Remainer arguments were also of the British Exceptionalist kind. Specifically the whole idea of “We should stay in the EU and change it from the inside” also reeks of “we know best” and “the EU will change to accomodate us”.