As thehappydrummer said, stick to Canon. You are use to the brand anyway, plus you most likely have lenses for yours. I would also consider keeping the 300D as a back-up/spare. Not much point in selling it.
A quick scan through eBay shows that the 30D is in and around £200 mark, and the 40D is £100 to £200 pound more. At Christmas you can get some good bargains, as people are eager to sell.
I would check out your local London Camera Exchange, they tend to do some 2nd user equipment and great prices on new kit. Also a store near me in Southampton called City Photographic (google them for website) is great for 2nd user kit. I've bought stuff from them before, including a Nikon D70 for my brother.
Good luck