That is a shocking response and absolutely not what you should expect from the supervisor on a flight. Did you take a photo of the seat by any chance?
If you have complained and received an inadequate response, you can escalate your complaint to the CAA. This is from their website.
Escalating your complaint
If you have complained to an airline or airport in writing and you're not satisfied with the outcome, contact the Civil Aviation Authority (CAA).
If you write to the CAA, make sure you include copies of all correspondence you have exchanged with the airline or airport.
Send your complaint to:
Passenger complaints
Civil Aviation Authority
CAA House
45-59 Kingsway
London
WC2B 6TE
Phone - 020 7453 6888
One thing I should advise. Be prepared to be specific about what you think is a reasonable remedy. I understand the CAA will ask you what you expect. If you received a return flight for example, and the impact was to make one of the flights uncomfortable, it is unlikely you will be able to demand a refund of the whole ticket or a free ticket. You should be clear with both the airline and the CAA what you think is a reasonable remedy.
You can also file a claim in small claims court. It is easy and cheap to do this, and it can be done entirely online at the link below. However, BA will certainly claim that you received the service you paid for, so you will have to establish that it fell below the standards a reasonable person could expect.
https://www.moneyclaim.gov.uk/web/mcol/welcome
It would infuriate me if a FA was as flippant as she was with you...but that may not rise to the level required in court.