Can I fix VMware Fusion to correctly map the tilde / back tick on a British keyboard?
Install VM Tools from the Fusion Menu (ignore notice about using open-vm-tools)
Select keyboard &amp; mouse from the VM settings
Highlight Profile - default on the list of profiles
From tools (cog below list of profiles) - select duplicate and name 'Debian Profile'
From tools - set profile as default
Double click profile and select Keyboard and Mouse
Make sure Enable Key Mappings is *checked
Make sure Language Specific Key Mappings is not checked
Use the '+' to add a new mapping of From: § To: `
Use the '+' key to add a new mapping of From: ` To: §

e.g. for the first mapping:

That should then take effect once you have a session running.
On the Debian side, you may have to configure the keyboard from the command line if you have not set it up / not running multiple languages.
First check if cat /etc/default/keyboard gives you:
# KEYBOARD CONFIGURATION FILE

# Consult the keyboard(5) manual page.

XKBMODEL=&quot;apple&quot;
XKBLAYOUT=&quot;gb&quot;
XKBVARIANT=&quot;mac&quot;
XKBOPTIONS=&quot;lv3:ralt_switch&quot;

BACKSPACE=&quot;guess&quot;

If so, ignore the rest and pat yourself on the back / get a cuppa :-)
If not install the command line configurator:
sudo apt install keyboard-configuration

Then execute it:
sudo dpkg-reconfigure keyboard-configuration

Select:

Keyboard Model: Apple
Keyboard Layout: English (UK) : English (UK, Macintosh)
AltGr: Right Alt (AltGr)
Compose Key: ???? - I do not use one - select as needed
Control+Alt+Backspace to terminate: ???? - your choice

Then REBOOT
The other way is to add the Keyboard Layout Handler into a panel BUT this sticks a big blob of text on your menu bar!  If you are happy with this (or are switching languages) then the UK keyboard can be set as:

Note the checkboxes for do not reset options and keep system layouts are clear.