Spend an hour and ring around your local photo dealers, they will take them in as part ex. Try ringing Jessops and ask have they got any at ANY branch across the country. If they have they will reserve and ship it to a branch local to you so that you can inspect it before buying (with no obligation).
You could always try posting wanted ads on the main photography sites including Pixalo? If someone sees that another person if after a 550 and they just happen to have one that they either don't use much or where thinking about upgrading to say a 580, your post may be enough to push them into starting a dialogue with you.
There is also one very big Canon specific forum that regularly has 550's offered in their members market but it has a worldwide readership, if the seller is foreign the postage could become an issue, never the less its still worth a punt.
There are many options if you really want to get your hands on a second hand 550, they are far from
Mythical 