Just to straighten out the question of sale of goods, any display or advert or offer for sale constitutes an invitation to treat, as stated previously. The offer is always made by the purchaser, and the vendor can choose whether or not to accept the offer. A pre-order is merely an advanced offer to purchase. No contract is made until the vendor accepts. Obviously if there has been a long delay, there is the good chance of a price rise. You can not expect shops to sell items at a loss - if they did not make a profit, they would not last long. What the shop can not do with a pre-order is charge you an increased price, without giving you the option of cancelling or accepting the increased price.
My own experience with Hattons relates to an item that took over 18 months from pre-order to delivery. They honoured the original price. I was only charged when the goods were despatched, as I would expect.
I think this has probably all been stated in different places before. My comments are based on what I can remember from my legal training.