Decades later the fucking OS still doesnt support the fucking layout. Luckily Liyang Hu is a badass and has made keybindings that work http://liyang.hu/osx-british.xhtml
I had nothing to do with this but I'm terrified that if that site goes down I won't be able to use a Mac (which sadly I have to for work and I have 25 years of muscle memory I don't fancy throwing away for Apple).
This repo is purely a backup. Please go to Liyang's site an heap praise on the hero.
move to the /Library/Keyboard Layouts folder. (Create the folder if it’s not there already.) You’ll need to log out and back in for the new layouts to become available. In certain situations you might find the ‘`’ and ‘\’ keys to be swapped, in which case the alternative layout is the one for you.
For me on Sierra `PC 105 alt was what worked for me.