As others have mentioned if your finger slips on the country drop down the charge vanishes.

The Government actually introduced Guidance and Legislation in order to stop excessive fees, which all low cost carriers now adhere to. No more than 2.5% fee and you cannot surprise people by suddenly adding it at the final payment page. You have to declare it at the start of the booking process.

I fail to understand why or how BA manage to flout the regulations, as they do neither of the above points. You don't know there is a fee until you select the card type.

I had an official response when I queried the fee back in 2014:

Thank you for your email dated 23 September 2014. Firstly, I would like to apologise for the delay in replying to you. I am sorry that you were disappointed about the increased amount of credit card surcharge. I understand this must have been very disappointing for you. Please accept my sincere apologies for any inconvenience caused.
We are aware of the Consumer Protection (Payment Surcharges) Regulations 2012 and are satisfied that British Airways are fully compliant. We set the credit card surcharge at a level which ensures customers are not charged more than the fees charged to British Airways for any given form of payment. The calculation is at an aggregate level and is averaged over card transactions. The credit card surcharge is designed to make a contribution to BA's payment fee costs.
Whenever you use your card, your card company charges the retailer a fee. In common with most other businesses, we have to charge the cost back to the customer. The fee you paid reflects the cost to British Airways of you choosing to book with your credit card. However, it only applies if you are using a UK billed credit card to book flights departing from the UK.
Overall, this helps us to reduce our distribution costs, so that we can continue to return savings to our customers through lower fares and special offers.
For further information regarding the way in which we set our fees, I suggest you to refer the advice given by the Department for Business Innovation and Skills by clicking on the following link.
http://www.gov.uk/government/organis...ovation-skills
As a valued customer, we appreciate your feedback. We assure you that your views would certainly be on our managers’ agenda for further improvement. Once again, thank you for contacting us. I hope this will not deter you from travelling with British Airways in future.