English opinion is very very strongly in favor of Scotland staying ... The question is, what is that reason. My conjecture is the reason is to be found in a certain wistfulness for the glorious past found in England.

It's not wistfulness for a glorious past, it's a present sense of national unity. There is no sense in England that Scotland is a separate country in a relevant sense. The country is Britain, and Scotland is a region of it with a slightly different history. In particular, there is no sense that England is a component entity within the UK or that there is "a Union"; the UK, Britain and England are all basically the same thing and the terms are used interchangeably.

This is emphatically not the case in Scotland (although it used to be, kinda). Scottish people now see Scotland as being a distinctive component entity within a "Union" with the other home countries, which has the right to decide its own fate. They deeply resent the English attitude as described in the paragraph above. They see Scotland being in the UK as a contingent decision. The worst among them also blame England for all their problems - Scottish nationalism is a very ugly thing.

Of course the UK government is broke, so they may well push the Scots to adapt a more austere position. But in these negotiations the ordinary English attitudes are a bargaining chip the Scots will use, not the English.

The issue isn't the government being broke (although that never helps). The issue is that English people have noticed the (changed) Scottish attitude in paragraph 2, and they do not like it. Surprise is now giving way to resentment, and in reaction, the English attitude in paragraph 1 has changed and is changing. Consider, for example, the most day-to-day way that people express their national attitude and identity - sport. 50 years ago (footage of the 1966 World Cup final is a good example), an English crowd cheering on an English team would be waving Union Jacks (i.e. the British flag) - because that's their national flag. That was typical at the time. Today, the crowd waves the George Cross (i.e. the specifically English flag). You're right that most English people want to keep Scotland in the union, but the fact that ~20% of English people want to get rid of Scotland is the truly remarkable thing. 50 years ago it would have been 2% at best.

Ordinary English attitudes are not a bargaining chip that the Scottish can use, because attempting to use them can only further undermine those very attitudes. It's like saying that a spouse feeling ambivalent about the marriage can get a better deal by using the feelings of the other spouse as a bargaining chip. No! Once you threaten to leave unless you get your way, and exposed your view of the marriage as a contingent one, you have poisoned their feelings towards you permanently. If you threaten to leave unless you get a new car, you are more likely to get your credit card cut off. Something broadly similar is happening in the UK. The more the Scottish ask what they can get out of the Union, the more English people demand that Scotland's privileges get revoked. Voters now back abolishing the Barnett formula and preventing Scottish MPs voting on English matters (see for example here ), and I suspect there is more to come, because the process now seems to be self-reinforcing.

Scotland leaving the UK would be a disaster for everyone.

I actually rather suspect it would be a mild boon for the rest of the UK, and (eventually) a great success for Scotland.