Why do most scientists think Brexit is bad for British science? From what I've seen in the media, almost all British scientists think Brexit is bad for British science. However I don't understand why. Some of the most common reasons I've seen are:

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.

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. Individuals might leave, but there's apparently no shortage of people wanting to move to the UK anyway (e.g. there were record numbers of international applicants to British universities in spite of Brexit). 

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.

Other reasons I've seen, like "there's no clarity on what we can or cannot do", should be temporary. They might be causing problems now, but five years in the future they should be resolved also.

Is there anything about Brexit that is fundamentally injurious to British science? If not, why do UK scientists so universally regard Brexit as grim?
tl;dr: Although it is possible to have Brexit without British science suffering, this would require meaningful political support, and there is little to suggest that this will be forthcoming in the near future.

A recent report by the Royal Society provides some context. This report is focussed on the British research sector as a whole, rather than on science specifically, but not too much turns on that. Some key figues include:


Britain receives a significant amount of funding from European bodies (e.g. ERC)
A sizeable fraction of the UK science workforce originates from other EU countries (16% of academic staff; 14% of PhD students)
70% of British researchers held a non-UK affiliation at some point in the period 1996-2011, although there is no EU-specific figure.


The overall picture is that British research benefits from EU membership, through direct financial input and through mobility of individuals. British universities are disproportionately well-regarded on the international stage (e.g. the Times Higher Education ranking has 7 British universities in the top 50, compared to 7 from the rest of the EU combined), and so historically Britain has been able to attract excellent researchers from around the world, especially EU neighbours.

It is right to say that Brexit does not need to change any of this. However, maintaining the status quo would require political and financial investment from the government, and there has been little evidence that this will be forthcoming in the short term - especially given the difficulties the government has had in agreeing the basic framework for Brexit. Domestic investment in research has fallen significantly in real terms in recent years, and it is not clear that any more cash will be available in the immediate aftermath of Brexit. 

It is also currently unclear how EU citizens currently living in the UK, and British citizens living in other EU countries will be affected. Some progress has been made on this, but (anecdotally) not enough to allay individuals' concerns. This is compounded by (real or perceived) xenophobic attitudes that precipitated the Brexit vote. Numerous UK-based, non-British researchers of my acquaintance are concerned over what the future holds for them, and have started to consider what other options are open to them.

Finally, it is perhaps worth noting that the Brexit vote was correlated with socioeconomic status, and the ideology of Brexit has never enjoyed much support in any of the academic circles I have been familiar with.