Since no one else has mentioned it, I can't imagine going that far out of the way just for a J seat to LHR. You could save a bunch more money by just flying C+ from the east coast. Fares out of MEX don't really seem all that much cheaper either. 60-day advance Z-fares out of ATL are available from $3900 roundtrip and it looks like there's a special in the middle of April for $2900 roundtrip.