How to change physical layout of the keyboard
OS X includes an British keyboard layout that you can switch to under Settings &gt; Keyboard &gt; Input Sources. If you hit the + button and search for British it'll come up. That'll give you Apple's version of a British keyboard layout.

If you'd like British to match a PC's keyboard, there's a second Apple mapping named British - PC.  This'll put the @ and | symbols where they should be in that layout.

You might find that Apple's British - PC keyboard mapping isn't quite right, as because their British keyboards aren't laid out as a typical British (BS4822) keyboard is. There's a great solution for this with a custom keyboard mapping. The one I linked there works like a charm.