It would be very interesting to see why they went back to LHR
Best guess - it's not unusual in similar situations for medics (possibly remote) to say that the medical condition(s) are not serious enough to necessitate an immediate landing but if an individuals condition deteriorates further a diversion may be required.
In the case of the AA flight I'd say that might well be a pretty compelling argument for not continuing westwards, and after discussion with "company" a decision was made to return to somewhere where the flight could be most easily "rebooted"....whilst retaining the option to drop into the likes of Kef, Glasgow or Manchester if required etc.
In any event it's perhaps also worth considering that given that we now know that no-one was admitted to hospital after landing so just maybe events weren't quite as dramatic as the papers would make us believe.