With regards to distance selling, the seller can even cancel the sale, AFTER, they have taken your money, if they stipulate when the contract is formed... Which most now do... at the point of dispatch, not check out. I know this because of an expriance with debemhas.... Never shop with them if you can help it!
In the uk, for a contract to be legal it needs to have "consideration" for both sides, I.e. be fair.... I'm sure you could argue a decent case for the point of dispatch not being fair for both sides....