I once had a violent blowout on the M6 near Leek and changed it myself but this was years ago and I was in my 20s, with no breakdown cover .it was extremely scary, with trucks whizzing by a few feet from your head. I wouldn't bother again. But anywhere relatively safe, yes, a 10 minute job at most, maybe less.

Managed to get a full sized spare from ebay for my partners 207SW for £65 but aftr weeks of nagging Ive failed to get my daughter to check whether her Mazda 3 has a spacesaver!

Too busy with everyday life I suppose!

Would a breakdown service have the exact make and size of tyre available when they come to fit?

No, and they would probably be unable to fit one if they had it. Needs pretty heavy equipment.

As for tyre availability i sat with my lorry beside the motorway for probably 6 hours following a trailer blow out one night, whilst the attending tyre fitter drove all over the district checking the depot stores for the right not that rare size, and there's dozens more car tyre sizes and types now than lorry....if you have a car the HA people would insist on recovery given that sort of time frame.

You cannot beat having your own spare wheel of some sort, even if its one of those horrid wheelbarrow tyres, it gets you to somewhere else, either home or to a tyre shop where you can get things sorted.

Changing your own wheel on the hard shoulder depends on the circumstances, there are places i would do it myself, and places where i'd want the HA people there with multiple flashing lights watching intently as they do for the increasing number of idiots that seem incapable of driving in a straight line.

What does make sense and few people bother, is to make sure before the event that you have all the equipment you need to change a wheel, and that it's serviceable, you know how and where to use it (jacking points), and that your wheelbrace will undo the bolts when the time comes, and lastly that you have the wheelnut locking key, a decent torch in case its dark, a coat and preferably a hivis in the car, the latter you'll be needing after the 29th of next month anyway :-)

Not aware of any change in regulations (Brexit or otherwise) that would legally require a hi vis to be carried?

I think GB might be referring to the likely protests of the yellow vests when Brexit goes belly up.