I find it hard to imagine it would take significantly more than 15 minutes for most people to ‘learn’ how to drive an automatic car even if they only drove a manual before.
Muscle memory is a bitch. Two things I’ve done going from a manual to an automatic, sometimes a year after:
Slam on the brake as if you’re using the clutch. Automatic brake pedals are more than wide enough to accommodate two feet, and attempting to upshift and slamming on the brake instead can really rattle your brain bucket.
Throw the automatic into park. Less of an issue these days, but it used to attempt to actually go into park, with the shenanigans you’d imagine there would be as the parking pin attempted to engage with the forcefully spinning gears.
I've never pulled that one but I've done a couple of very hard stops coming off the freeway. Long drive, tired, I hear the engine RPMs dropping and my foot comes down on the "clutch" as my hand goes for the gearshift. I learned on a car with a very stiff clutch, I catch the brake pedal and it's going to the floor. (My father had a bad habit of riding the clutch while going through the gears. A mechanic friend deliberately put in a stiffer spring to minimize the damage.)
Smoothly operating a manual has to be muscle memory and whenever you're dealing with muscle memory there's the issue of unintended capture. It doesn't take much to reprogram things like learning the actual working range of *this* clutch but skipping it entirely takes a lot more learning.
Moving from manual to power brakes is also a recipe for slamming on the brakes..
In high school, I drove a really old car with manual everything (including brakes). Our drivers ed class was taught on 80s american sedans, with overboosted power steering and brakes where you could probably just touch them with a single toe to stop. Coming from the manual brakes in my car where I needed to stand on them to stop, I'd inadvertently slam on the brakes early in my turn at the wheel and throw everyone against the seatbelt tensioners..
I once had power brakes fail in my car, (vacuum line leak, rubber connections disintegrated with age, thus: no turbo and no brakes) but needed to go on 300 km trip (in hindsight, I contracted get-there-itis and this was unnecessary risk). At first: woah, I have no brakes! But after few minutes I got used that I need to stand on the pedal and it somewhat braked. But it was surprising when I finally replaced that rubber piece: obviously I knew about this, but for first few stops I slammed the brakes hard anyway.
The UK authorities agree with you: if you pass the exam with a manual car you can use both, but if you pass it with an automatic one, you can only drive automatic ones.
I'm not saying it's hard (I had to, given the automatic rental when visiting the US), but it's not just about learning how to do it, it's also about reflexes, which you don't unlearn in 15 minutes. Sure you can learn to consciously suppress them, but the habit of finding a pedal with your left foot when accelerating/decelerating lures behind it for a lot longer.
