Companies must honour the price stated at purchase – if they then change the price after order process (that is to say when you did your bit on the internet site) then they must honour this by law, even if the next day they are charging double the price.
Similarly if you’re in a supermarket and pick up an item which says it’s £5 then take it to the checkout and they say, it’s £10 then they must by law honour the price at the advertised price of £5 (you might have to take them to the shelf to prove this though). I know it’s not excatly the same thing, but the point is they must honour the price you bought the item at (the advertised price when you wanted to buy it) even if it’s not dispatched straight away.