You need to run the Keyboard Setup Assistant again, and the only reliable way to do that is to delete the files it generated as follows (run this in the terminal):
sudo rm /Library/Preferences/com.apple.keyboard.plist
Then restart your computer. When you log in / plug in the keyboard it should show you the Keyboard Setup Assistant and ask you to press the key to the right of the left shift, i.e. the
\ key on a normal UK keyboard. Do as it asks, and it should detect your keyboard as ISO (not ANSI). Accept that, and then make sure in
System Preferences -> Keyboard -> Input Sources you are using the
British - PC layout.
That should give you a normal keyboard layout, with one exception - the
Ctrl key (the bottom left one) will be mapped to the Mac
Control key, which you rarely use, and the Windows key will be mapped to
Command. You probably want to swap those so that copy/paste shortcuts etc. are the same as on Windows.
The easiest way to do that is to install Karabiner-Elements and set it up like this:
Also note that this keyboard layout is not applied before you log in after a reboot, but it is applied before you log in after logging out. Very confusing if you have
@ in your password!
Edit: I have found an easier way to re-run the keyboard detection wizard rather than restarting - change the Country Code in Karabiner-Elements as follows:
You just need to increment it. The actual value doesn't matter.