Apple - How switch Mac UK PC keyboard layout backslash \ and backtick ` to match normal UK PC layout

It appears to be controlled by what you actually have connected... but there is something else going on I'm still trying to hone down.
The best conclusion I can come to at the moment is "it's a bug" but one that seems to be at least partially fixed in High Sierra & Mojave.

This is a UK Apple Keyboard on Mojave [confirmed identical on High Sierrs]

When you first open the panel, it shows as ANSI.

Press Shift [which I discovered by accident trying to take screenshots] & it changes to ISO.

Then add British PC & it seems to retain that information...

Now I've persuaded it to show like that I can't 'break' it again, it seems to stick so far.

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However, testing on El Capitan, I can't persuade it to flip to ISO, no matter what I do - it stays as ANSI...

Test on the El Cap machine, swapping a TextEdit document from British to British PC - even though the control panel still claims it's ANSI & after clearing all keyboard prefs & re-detecting the keyboard...

The Mac 'knows' it's ISO, but won't display as that in the Keyboard Input Sources control panel.

You seem to have a US (ANSI) keyboard, where there is no letter key to the left of the Z. Is that correct? For a "normal" UK layout, you must have a European (ISO) keyboard which has this extra key.

If you do have an ISO keyboard with extra key, then you have a keyboard type problem.

In the end, installing the Karabiner app fixed the issue.