Seems to me that neither you nor BA can have it both ways. It’s either a disability, in which case the entire party becomes ineligible for exit row seating but any other seat selection is free, or it’s not a disability, in which case members of the party are able to sit in the exit row but that seat selection has to be paid for

since we know that in many/most cases autism is a disability, it has to be option a. You may not agree with the policy, you may think the rationale is flawed, but that is the policy as published, and is one that extends across many airlines