http://www.britishairways.com/travel...g/public/en_us
• Executive Club Gold members, oneworld Emerald member equivalents and other passengers on the same booking can request an exit row seat free of charge, at any time before the flight.
• Exit row seats can be pre-assigned at a cost of £50, €60 or US$75 per person per sector. This fee will be converted into local currency where applicable.
So, it appears correct that you were asked to pay the fee for pre-assigned exit row seat