It's not the UK, but in Hong Kong on iOS 15, Apple Maps now automatically taps you on the wrist (apple watch) right before it's time to get off the public bus at your stop. It is freakishly accurate.
It's a very nice feature. I'm not quite sure of what metric they're using. Some busses can show real-time locations, others they obviously dont have that info. Sometimes it will ping me a stop early (if the stops are close together?). I do wish there was a way to either stop directions on the phone, or put it in low power mode. It seems to eat my battery on long bus trips if its just sitting in my pocket. On car trips with directions I'm often charging the phone. On the bus or when walking I'm often not.
In practice, there are still some annoying wrinkles. It often assumes I boarded a bus when I'm not quite to the bus stop. So I have to tap and swipe a few times to go back, find out which platform or when the next bus arrives. If you have a moderate walk to or from the bus stop, the walking directions are frankly bad. I often quit and redo that part as only walking to get turn by turn. I know the watch has a much more limited screen, but I end up having to pull out my phone for some of those specific details like the platform or when the next bus is scheduled.
That might work for a bus out in the open, but GPS doesn't work well with a lot of nearby tall buildings, or for subways underground.
Maybe they are using GPS and silently failing in all other situations? My point is that it's nice when it works but could use some improvement on whatever method they're using.
This might be true if you're walking up a hill but its not true if you're trying to go to a specific place (restaurant/shop etc) with opening times via any mode of transport.