Loss of funding - UK researchers receive lots of funds from EU grants, and these would not last once the UK leaves. But since the UK pays the EU more than it receives in benefits, the UK could simply redirect some of that money into science. It could be that the UK does not actually redirect the money, but that would be because the UK as a whole decided that science isn't worth it, in which case it would be democracy in action and one can't really complain.
The Brexiter's solution to everything. I thought this extra money was supposed to be used for the NHS? Or that it was supposed to be used to help British farmers/workers to help with the new tariffs with the EU? Even the proponents of Brexit had to admit that this was a lie, and that you cannot evaluate the repercussions of such a huge decision with a pocket calculator and two numbers. The fact is, nobody knows how research funding will be affected by Brexit, which has real consequences now: who will want to make a life commitment to go work in the UK when they don't know what their situation will be in 5 years?
The UK gets a lot of money for research through the ERC. Nobody knows how that will work out after Brexit. Only two non-EU countries participate in it – Switzerland and Israel, and two years ago it was not even clear if Switzerland would be allowed to stay in due to concerns regarding free movement of people. Compare this with the preoccupations of scientists, who are concerned whether they will be able to continue their research projects over the next ten years, hire new students, postdocs... Why should all this hinge on politics when you can play it safe and go work in an EU country?
Isolation - this seems to be based on the idea that Brexit makes the UK a less attractive place to work in. However the US didn't seem to suffer a similar brain drain after the 2016 elections, so it's not obvious to me that the UK will. Besides, there were record numbers of applicants to British universities in spite of Brexit.
I think I covered this extensively above. Let me add that issues regarding visas are also problematic. Applying for academic positions is already an ordeal, if one needs to apply for a visa afterwards with no certainty of getting it, it becomes hell. You may argue that, maybe, there will be deals to make sure that people from the EU can continue to work in the UK and get easy visas, but that's the thing – we don't know. Can you tell me that you would be willing to uproot your life, move to a new country, all the while not being sure whether you will be allowed to remain in the country based on the whims of ten DUP members of parliament?
The situation is also not at all comparable with the US elections. The elections are reversible; at the end of the year part of their congress is replaced, and in two years maybe this whole thing will be over. Brexit is a priori irreversible. Moreover, the general sentiment I feel when talking to people is that incompetence at the top prevents too drastic changes. Here, incompetence would mean the opposite – maximum change.
Loss of collaborations - it's not clear to me why this would happen. The EU presumably doesn't have a "you may only collaborate with other EU researchers" rule, since there're lots of countries that are not part of the EU. But if they don't have such a rule then presumably collaborations can persist whether or not the UK is within the EU. Further, the EU would be incentivized to keep such collaborations because they're presumably win-win. Finally, even if EU collaborations are jeopardized, there's still the rest of the world. There are scientific powerhouses like the US that aren't part of the EU either.
You have to understand how research is funded. The most attractive grants are collaborative. You apply for grants together with people from other institutions/countries, and then you get money to run experiments, organize workshops together, visit the other institutions part of the grant to learn from your collaborators and actually work in the same room...
Once again, it is unclear how Brexit will affect all this. Given a funding source and British partners, will it still be possible after Brexit to either keep it, or apply again for it? Nobody knows. Since applying for these grants is so time-consuming and the success rate so low, why would you risk it all on the bet that politicians will come to an agreement that suits you?
As for "the rest of the world", well, it's certainly easier to work with people who are a two-hour train ride away than a transatlantic flight away. And yes, the US is a powerhouse, which may not be as good for UK as you seem to think: why would US scientists bother to apply for grants with British scientists (from which funding source?!), when there are perfectly good collaborators already on the same continent working with the same funding sources (NSF etc)?
There is also the question of the timeline. You say that the situation is "temporary" and that "five years in the future [issues] should be resolved". One problem, as outlined above, is that we don't know if the situation will be resolved, or how. Perhaps the UK government will manage a complete upheaval of their research funding systems to not rely on external sources after 45 years of integration with the EU. Maybe not.
But you're forgetting about the irreversible effects that happen now. Students want to enter PhD programs. New PhDs want to find a postdoc. Postdocs want to find faculty jobs. Less funding and less certainty about the future means that these people either will not find a job in the UK and leave academia (if they are bent on staying in the UK) or apply for jobs elsewhere. And once you start working in some place, you're more likely to stay there, simply because you start to know the "system" better and you meet more people from that place.
Of course, I paint a pretty bad picture of the situation above. British academia will not collapse overnight on March 29th, 2019. But it will certainly be in a worse shape than today. And given the very competitive world we work in, everyone needs any edge they can get.
Finally, let me mention that two Nobel prize laureates have claimed that research will take a blow.