[1] https://commons.wikimedia.org/wiki/File:KB_United_Kingdom.sv...
It also doesn't have the wonderful set of Alt+ combinations as the Mac British layout for typing various types of diacritics, widths of dashes, Greek letters, mathematical symbols, opening and closing quotes, and other things. PC layout users are stuck with acute accents, the Euro symbol, and nothing else.
Luckily, the Mac "British - PC" layout (not the same as the actual British PC layout) does include the extra Alt+ combos.
I'm not sure why anyone would find the Mac British layout difficult to program with. It's closer to the US layout than the PC British layout. I suppose # requiring Alt might be bothersome, but # isn't a symbol used that frequently in programming, and Alt+3 is not that difficult to type.
Also, I use # loads every day for c++ macros, and #if 0 blocks.
Apple's had their own layout for literally decades. There wasn't a standard back then.
> Having always used a standard british keyboard, the subtle differences, e.g. location of \, #, ", `, ~, really slow me down.
You can adjust to it pretty quickly.
My habit when I have to use a Mac is to switch to the Australian mapping, which is the same as the British one but with £ and # switched.
I have them both mapped to British layout and my fingers just "know" where things are. However, my conscious brain might not. I actually had to think really hard about where the '#' key is. On the American layout, it's printed above the number 3 (Shift-3), but actually produces the '£' symbol. The hash is above the return key and is printed as '\'. On the German keyboard it's actually in the "right" place, just to the left of the return key.
The biggest key I have problems with switching between American and British keyboards is the @ symbol. If I don't remap the keyboards, I'm forever typing " or @ incorrectly. On un-remapped German keyboards it's Y and Z.
I still try to buy my personal laptops from the UK, so I have the correct keyboard. It's a surprisingly big productivity loss to have incorrectly mapped keyboards.
Also, there's a nice single-serving site for your Q: http://wheresthehashkeyonamac.co.uk