Go to this site: http://www.faxyourmp.com/
Put in your postal code, and up pops your local MP.
Write your letter, then click the link in the email they send you, and voila, your fax is sent!
Does it work? Don't ask me, I don't live there anymore! But here's some press on it: http://www.faxyourmp.com/press.php3
Expatbrit