You can't insist no. And I can't see them rebooking your on to Aeroméxico. They will rebook you via AA or Iberia however, if there is space. And they will offer a full refund which may help you rebook on another airline. And you can also ask BA to move to other dates, within a -3 days to +14 days margin as a standard offer, and for a wider period under the Buy with Confidence policy.