AIUI the reason appliances in the UK used to be supplied with a bare cable is that it avoids the need for extra SKUs. If you sell a Washing Machine which can do anywhere from 100 to 250V AC input, you can make 5000 of them, and you don't care if 50 or 500 of those sell in England, it's all the same to you since it's the same SKU. If the English ones need BS1363 plugs (as they do today) then that's a separate SKU.
The reason it changed is that it turns out a ludicrous number of people can't safely and correctly wire a plug. On older circuits if you wire Earth where Neutral should go, the appliance will probably work, but now you've significantly increased the risk of faults becoming fatal. Also UK plugs are fused and left to their own devices almost everybody will put a 13A fuse in the plug (the maximum size commonly available and equivalent to the maximum ~3kW energy draw for 240V home mains) even though very few appliances need that. So now when there's a fault and your 5W bedside clock radio is drawing 2kW (plenty of energy to start a fire), the fuse doesn't blow and instead your house burns down. It's very easy to tell manufacturers "Put the right size fuse in" because they're ordering crates of fuses they can just order the correct one, but home owners know the 13A fuse will "work" so they're not going to buy a 5A fuse instead and risk finding they've got the wrong kind.
As a nerd kid I cared a lot about this, and I would maybe find about half of appliances I was allowed to examine were wrong in some way, most often over-size fuse but sometimes loose wiring or even just straight up wrongly wired. Today those appliances have a pre-installed plug which is almost certain to have the right size fuse and be correctly wired just because it was done as a production line task so somebody was actually trained and given the appropriate equipment.
The reason it changed is that it turns out a ludicrous number of people can't safely and correctly wire a plug. On older circuits if you wire Earth where Neutral should go, the appliance will probably work, but now you've significantly increased the risk of faults becoming fatal. Also UK plugs are fused and left to their own devices almost everybody will put a 13A fuse in the plug (the maximum size commonly available and equivalent to the maximum ~3kW energy draw for 240V home mains) even though very few appliances need that. So now when there's a fault and your 5W bedside clock radio is drawing 2kW (plenty of energy to start a fire), the fuse doesn't blow and instead your house burns down. It's very easy to tell manufacturers "Put the right size fuse in" because they're ordering crates of fuses they can just order the correct one, but home owners know the 13A fuse will "work" so they're not going to buy a 5A fuse instead and risk finding they've got the wrong kind.
As a nerd kid I cared a lot about this, and I would maybe find about half of appliances I was allowed to examine were wrong in some way, most often over-size fuse but sometimes loose wiring or even just straight up wrongly wired. Today those appliances have a pre-installed plug which is almost certain to have the right size fuse and be correctly wired just because it was done as a production line task so somebody was actually trained and given the appropriate equipment.