The T&Cs of most UK based big e-commerce now seem to include phrases stating that there is no contract between you and them until they have taken your money. I believe this has been tested in court and found to be valid. I'm too lazy to try and dig out any proof of this though...

I have recently used the new Distance Selling Regulations to good effect though. I managed to get £340 back from scan.co.uk after deciding I didn't like the monitor I bought from them. They took £24 to cover the postage there and back, which I think is reasonable for a 30kg package.