Originally Posted by NickB

Most credit card surcharges are banned in the UK but Air France and KLM seem to have found a loophole in relation to Amex cards and exploit it to their hearts content: see here for a discussion of the issue. I am not entirely sure whether their position is still legally correct following recent judgments of the ECJ on interchange fees but it is too much hassle to investigate and challenge.

You should not be charged a fee if you use a Mastercard or Visa, however (unless perhaps if it is a corporate card).