How can I type the pound/hash (#) symbol on a British keyboard when in Terminal?

If the only issue you have is that is that Shift-3 should be # then a solution is to change the input source to Australian rather than British.

This is a duplicate of my answer on Stack Overflow

I assume that you have a Mac UK keyboard so Shift-3 is £. On most other keyboards Shift-3 is # as others have said.

The way I get round it is to change the input source to Australian the only difference is that Shift-3 is now # and Alt-3 is £ (or leave as the emacs binding)

Input Source setting was System Preferences->Language&text->Input Source
On later OSX versions (OSX 10.11 definitely but would have been earlier) 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

ABC and US and Canadian English are identical to Australian, if you prefer them. Irish should work too, but it is different on the Option level and has no £.

To quickly change Input Source, use ctrl+space. Holding ctrl after lifting space bar shows the list of all added keyboards.

That is the real pound symbol ;)

The hashtag one # is opt/3 on a British keyboard layout.
If you want the US layout, you'll have to swap to US English.