The Electronic Commerce Regulations 2002 cover internet transactions. It modifies what was traditionally considered as the point at which a contract is formed.
In a shop the "invitation to treat" is followed by "acceptance" at which point a contract may be agreed.
ECR 2002 allows that "acceptence" (email, booking confirmation, receipt of payment, etc.) need not be the point at which a contract is formed but can be as late as despatch or supply of the goods/service, thus all European internet transactions have the flexibility for price changes. This "pre-contract" information must form part of the Terms & Conditions of the website.