Shift+3 returns £ sign instead of # sign
The keyboard is working as designed for Apple.

If you want to use the keyboard as normal the # is given by Alt-3

I found this annoying as well and the way I get round it is to change the input source to Australian the only difference is that Shift3 is now # and Alt3 is £ 

Input Source setting is System Preferences->Keyboard->Input Source By default this will just show the UK keyboard to see more hit the + at the bottom of the list and add Australian

This is the answer I gave in Stack Overflow and I think is a duplicate here as well but I could not find the original.