Another shout for a stopover in cdmx. Courtyard Marriott is a great hotel at the airport and then Uber into the city. Check out and walk into t1 for your BA flight. With regards to a same day flight make sure you look at what terminal you fly into. Aeromexico is t2 so you have to transfer between terminals for your BA flight. Viva aerobus is an option for t1. Also make sure you are going to the right airport now they have opened the new one north of the city!