Once again, follow the instructions in my first two paragraphs above, then . . .
. . . under 'Preferred languages', click on 'Add a language'.
In the box that appears at the top of the panel, type 'English'.
Scroll down to 'English (United Kingdom)' and click on it.
Click on 'Next', then on 'Install'.
Wait for the word 'Downloading', near the foot of your screen, to disappear. (It could take several minutes, during which time nothing much will seem to happen. Don't panic, just give it time!).
Then use your mouse to drag 'English (United Kingdom)' up above 'English (USA)', so that it's first in the list.
The entry beneath 'Keyboard' should then read 'English (United Kingdom)' and your keyboard should then function exactly as its meant to.