Consumer Guarantees Act 1993, s 11
If a price has not already been agreed, you do not have to pay the seller more than a reasonable price for the goods you are purchasing.
A reasonable price could be determined by what other sellers charge.
If the price is unreasonable, you can refuse to pay more than a reasonable price. If you have already paid for the goods, it is too late and you do not have a right to a refund.