Another issue with this scheme that I didn't think of before but realized now: scientists mostly get the greatest prizes once they are quite advanced in their career. The Nobels are infamous for being given out decades after the fact to people in their 60s-70s-80s, of course, and it's not that bad for all major prizes; but they are general for very senior scientists that are in the final stretch. That's when these scientists are least likely to move: they are probably already at a very highly regarded institution with all kinds of privileges due to their status, will have a well-running research group, likely have well-established social lives, may have kids, and so on.Right but UK science is genuinely amongst the best in the world. That isn't some verbal bluster, nor will it necessarily attract a German/French/Chinese Nobel laureate but its an undisputed fact.
Of course, that doesn't also mean that the Tories aren't talking nonsense or that they're wrong to link it to some kind of stupid 'points based immigration' they can only have because of Brexit. They could of course have done the same in the EU too.
Scientists don't spend their time going around interacting with the population at large, discussing politics. A scientist who decides to come to Imperial or Cambridge is employed directly by the university and will be funded for their research by the university, so I'm not sure what relevance opportunism will have in their actual day to day life. Same for a scientist at MIT or Sorbonne. I'm going to assume that there isn't much logic or reasoning in Trump's election or Xi's cult of personality or France's lurch to the right with Macron trying to outflank Le Pen on Islam or Japan's essentially one party rule for the last 60 years. India and Russia tend to be pretty high up on these lists too.
I think there's much than enough things to blame on Brexit, as well as more than enough reasons to shy away from moving to the UK, without trying to bring the logic or reasoning of the general populace into play I think. As much as its tempting post Brexit to stick the boot in in every way.
Long story short: from what I know about scientists, if you want to start a scheme to attract the best and brightest, you go for the people that are getting super high recognition in their early careers - during their postdoc or first appointment. Many won't give care a bit about societal circumstances as long as their work conditions are good. (I.e., reduced access to EU funding isn't popular with them, but Brexit or xenophobia won't deter them.)