I'm not sure this situation qualifies for a full refund, as BA does not differentiate seats 39A/B from those elsewhere in the cabin (other than exit row seats which are bookable in advance at a higher tariff)

From BA.com: Paid Seating Terms and Conditions - Refunds

• Paid seating is non-refundable unless (a) there is an aircraft change and BA is unable to seat you in a suitable alternative seat. (b) there is a flight disruption causing you to be moved to another flight and BA is unable to seat you in a suitable alternative seat on your new flight.
• Paid seating will not be refunded if you cancel your flight, are involuntarily upgraded due to oversales, or are not suitable to sit in the seat type you have selected.
• If, in accordance with your fare rules, you choose to move to a different flight, you will be entitled to choose an equivalent seat on your new flight. However if an equivalent seat is not available the additional amount paid will be forfeited and will not be refunded.
• You have 14 days after the last flight in your itinerary to apply for a refund.
• Refund applications should be submitted online. Refund applications cannot be processed at the airport or onboard.

https://www.britishairways.com/trave...g/public/en_gb

If you dissect the rule i've underlined above, you may be able to construct an argument along the lines you booked a window and an aisle seat; that as a result of the aircraft change, the aisle seat was changed to a middle seat; that you do not consider a middle seat to be a suitable alternative to an aisle seat. On that basis, you may have valid grounds for a refund but only in respect to seat 39B.