MacBook Air: Change dollar to pound symbol on key 4 in UK

The simplest thing is probably just remember that pressing Option-3 will produce the £ sign anywhere you type.

You can change the keyboard to the British keyboard. Go to System Settings > Keyboard > Input Sources. Click the "+" to add a new Input Source. Select English > British > Add. That should remap your keyboard to the UK.

To change your keyboard layout, please enter the settings application, scroll down to keyboard in the sidebar, look at text input -> input sources. There you can add the English - British keyboard layout, and, by chance, remove your apparent standard/US one if you no longer require it.

Nonetheless, the British keyboard will still have the shift+4 input as '$' and I currently see no way to mitigate that.