Never been on an MKII or an MKIII that doesn't have powered doors?
Open the window, reach outside, turn the handle and away you go!
Not that I can recall, when I was a kid we always got to push the buttons top open the train doors, of course when on the older stock our parents may have known they weren't unintuitive and took charge without us realising. Even then the bulk of the stock we were travelling on was Sprinters, Pacers and Networkers (i.e. mainly local services from Preston and Oxford in the mid-late 1990s)
By the time I was travelling on my own it was Pacers and Sprinters to Manchester or Voygers/Pendalinos to Crewe and then Turbostars to Nottingham. Somehow managed to avoid any slam door stock (where I was at the front of the queue anyway) for 15 years!
Then again, when I was a kid buses used to have a pole in the middle of the stepped doorway, and most shops had steps to get in the narrow doorway too - Can't remember the last time I got on a bus which didn't have a wide entrance, low floor, and no pole in the doorway, and it's rare for me to encounter a shop with a stepped entrance too. Things change and times move on - and those door handles on a HST (coupled with the bouncy ride, fixed armrests and general tattyness of the EMT fleet) make the HSTs feel like a relic of the past.
Old ≠bad design.
The design of having the handle on the outside pre-dates central locking and push-button doors. And there's a very practical reason why the handle on the outside is a safer design - it forces the user to at least look out of the window, reducing the likelihood of opening the door while the train is moving, or where there is no platform.
That's just reminded me of the rest of that bit of the presentation - I do remember him (an American (I think) UX expert who'd struggled to get the doors open too) saying it made sense originally (i.e. if you open mid journey you fall out and die), but with central locking it's just unintuitive. Another bad bit of design he featured was the plug sockets on the train - mounted in such a way that if you had a Macbook power brick you couldn't plug it in as it was mounted so near the table top!