1. Switch on the "locale" module at admin/build/modules
2. Go to admin/settings/language/add, add your language using the "Custom Language" drop-down. Sane defaults are:
   
   1. Language code: en-GB
   2. Language name in English: British English
   3. Native language name: English

Might it be a good idea to have a word list?

Use this wiki page to add things, eg

Prefixes and Suffixes

-ize/-yze

Change to -ise/-yse. (e.g. synchronise, analyse)

-or

Change to -our. (e.g. colour, neighbour)

-er

Change to -re. (e.g. centre, metre)

-g

Change to -gue. (e.g. Catalogue, dialogue)

-l-

Change to -ll-. (e.g. enrollment, modelling)

-ed

Change to -t. (e.g. leapt, learnt) (This is not always the rule!)

This wiki page should be instructions on how to set Drupal up to use British English translations. This is marginally more complicated than most, as Drupal by default doesn't know the en-gb language code and has to be told about it.

Some basic bits and pieces can be culled from my blog post here: http://www.drupaler.co.uk/blog/translation-proving-complicated/455

The rest could do with someone more clued up on this than me!