MCT is enforced and AA won't be able to book an illegal connection. But MCT at LHR is seasonal and changes every few months as various systems at LHR are improved, so the date of your trip determines what MCT is required. There were about 20 different MCT values at LHR during 2008 and probably the same next year (further complicated by the BA T1 and T4 moves which keep getting delayed). Once all of T5 is fully operational (if ever), the MCT should be more like 45 min -- it was meant to be 30 min. HEL manages 20 min MCT today, so that isn't an impossible goal -- just very unlikely.