I don't think it would be a problem. Any informed potential UK bond-buyer / investor will be well aware that the ROSCOs got hold of a load of stuff (almost all our rolling stock) which gave them a strangle-hold over British railway TOCs at a bargain price, and astonishingly have continued to hold the country to ransome for the use of (mostly) the same old stuff ever since! Merseyrail buying its own trains showed them the way things were going to go from now on...
The right-wing press will scream about it, but "the markets" will recognise that they have had an unbelievably easy and lucrative ride for a few decades, shrug their shoulders and move on to some new opportunity.
You might think it won't but a problem, but I disagree. 30 years on, the pre-privatisation fleet is a small proportion of the total, and the ROSCOs are not holding the "country to ransom", they are keeping to contracts freely entered into on both sides. Most of the current fleet is new stock on agreements struck when long term interest rates were low.
The institutions that currently own the ROSCOs are not the people who "got a load of stuff at a bargain price". Those institutions don't just own the ROSCOs, they have other UK investments too, and the government wants them to invest more. They are not interested in the past, only the future.
If the UK acted in the way you suggest, then the new opportunities for them to move into would not be in the UK, they would take their money elsewhere.
The UK rolling stock market is much too route specific and bespoke to allow a true market to operate, unlike the market for airliners, for example, which can be reused almost anywhere around the world.
You are right that the market for airliners is much more liquid, but the UK rail market being "bespoke" doesn't mean that leasing can't work, for new stock there is still competition between suppliers. If the UK government was in a better position to borrow, then that would be an option too.
Hence you can end up with electric fleets being scrapped prematurely while old diesel stock continues to be paid handsomely for.
The problem there isn't leasing. New diesel trains couldn't have been financed in the 2010s, in the same way as electric fleets, because the move towards net zero meant that they would not have had a 30-40 year long life.
I don't think passengers did badly out of the HST life extension at all.
A life extension under lease.
why should the farepayer/taxpayer be stuck with those consequences.
It was and will be forever thus. We all have to live with the consequences of government decisions, sometimes for a long time after they are made. One of the lessons from 2 years ago!