UK.keylayout lets you modify UK keyboard mappings.
The standard British key layout makes a UK-issue Apple keyboard work as it should do, but for some of us who have typed on non-Apple keyboards our whole lives - or for those Mac owners who are using a third-party keyboard - the Apple standard layout doesn't feel quite right. For example, Shift+2 gives you @, whereas on more standard keyboards it gives you ".
This file will map the " @ # ~ | £ ` keys to be where you would expect them to be on a UK keyboard.
Option+3 -> #, Option+§ -> `(grave or back-tick)