Another important consideration is the accounting. Since the UK as a whole has a substantial annual deficit at the moment (and for the foreseeable future), spending on higher education isn’t paid for out of general taxation (on the margin) it’s paid for through the public sector borrowing requirement. This bothers the UK Government much more than it bothers the Scottish Government. So to the UK Government it’s very attractive to shift public spending from adding to the national debt to any smoke-and-mirrors accounting scheme you can find that makes it look like it doesn’t. Oh, and totally legitimate schemes that have the same effect are even more attractive, of course.
I strongly suspect this is at least a big factor in why the sold-off loans approach is favoured by the UK Government but not by the devolved administrations. This suspicion cashes out as a prediction: an independent Scottish Government is likely to be keener on such schemes than a Scottish Government that remains part of the UK.