I’m not sure I understand the bit about the 3% fee on the refund as the refund should be increased by 3% not reduced by 3%.
ie 1150 EUR = £1030 of it was 1.15EUR/£ when you bought and £1076 if it was 1.1EUR/£ when refunded.
Didn’t all the Avios transfer over into your actual BAEC account?
If your card is now in a negative Avios balance just cancel the card and get a refund of any remaining fee. This sign up at some point in the future when you might be booking BA flights.