This would appear as to why the OP was charged £9.

From ba.com

You can pay for your booking online with a credit card, charge card or a PayPal account for bookings originating in most countries. Payment must be made in full.

If you are travelling from the UK, you can also pay with a Switch/Maestro, Solo, Visa/Delta debit card issued by a UK bank and billed to a UK address.

There will be no charge for paying with a debit card, but there is charge of £4.50 per ticket if you use a credit card or PayPal.

http://www.britishairways.com/travel...?refevent=help

Page 2