I just checked the British Airways website for tickets purchased in Mexico, one way Mexico City to London, departing 8-Sep on BA-0242. The website quoted US$1643 which is a whole lot closer to your 900 pounds price than US$300. This is a non-stop routing.
ITA fare search (
http://matrix.itasoftware.com) for MEX-LON 8-SEP one way came up with a low fare of 9015.41 Mexican Pesos (US$791.51) for a route MEX-JFK-DUB-LHR on Mexicana Airlines and Aer Lingus. Again, this would be a fare for a ticket purchased in Mexico. Same ticket, purchased in London, comes up as a fare of 428 British Pounds (US$792.53). No material difference in fares.