I know I'm writing this on a tool that doesn't have a British English dictionary (this will come very soon) but Microsoft in partnership with the British Library are very keen to improve this and want to create a series of online dictionaries for Office 2007 that reflect our colourful and diverse local dialects.
So who are they asking to help them? Why us! The British. It's our change to contribute. So if you have any suggestions then send you contributions to them via the Local Dialect site.