The you talk about the UK not surviving this, I assume you mean losing Scotland and NI. The average Englander won't be bothered by that and will indeed incite more nationalism which works against rejoining.
I see it the other way round.
Should Scotland get independence, and should they then pursue EU membership, it would put England/Wales in an ever more isolated position, it would also highlight to the English even clearer the real problems of the TCAA and how it inhibits trade, as it would be quite difficult to implement the English/Scottish border in that respect.
It could however make Scotland an attractive place for business, being the only EU state with a land border with England. We have seen much UK business move to EU countries for distribution purposes into EU, it's not wild to consider Scotland becoming an attractive Single market territory to distribute EU produce into England.
From Scotland's point of view it would also likely mean they could not join Schengen, so a CTA would have to be in place between Scotland and England/Wales, just like the ROI one.
However that would put an EU based Scotland in a very powerful position should England/Wales rejoin after, as they could, in theory, insist upon Schengen adoption for England if they want to rejoin, effectively enabling the whole of the British Isles to become full Schengen members for the first time.
As for the nationalism angle, just like every country around the world, that will always be there, however there has been a significant change in the last few years. Before Brexit, leavers could use emotion as their main weapon against EU membership. That has now changed. now they have their Brexit they are now having to reason with facts, which aren't really in their favor. There will always be outliers who insist this is not a "proper brexit" and such to deflect the realities, but in my view this is a shrinking segment.
As ever it will be the media barons who ultimately control much of when things happen. At some point their financial backers will have milked brexit for all its worth to them, and could move on to the next thing. Watching the evolving EU narrative in the press will be interesting.
There have already been some isolated articles in Telegraph for example which have admitted failure, and looked at the wider picture. For me these are the signposts we need to watch.