Is this for X or for console use?
In X, the quickest way is to open a terminal, su to root, then edit /etc/X11/xorg.conf
Look for the keyboard section, and change it to the below:
Option "XkbLayout" "gb" ## KEYBOARD_MAP!
Option "XkbModel" "pc104" ## KEYBOARD_MODEL!
Option "Xkbvariant" "" ## KEYBOARD_VARIANT!
If it is set to US, you will find the 'Option "XkbLayout" will be us, not gb.
Whoops. Forgot. To change the layout for console, you need to edit (as root, of course), /etc/rc.d/rc.keymap. In this instance, the correct value for loadkeys is 'uk'