But, not knowing what the cause was, and therefore the possibility that more crew (including FD crew) and passengers could become ill, why all the way back to LHR? That decision puzzles me.
Would the need for specialist testing require LHR landing?
Could have been minor at first prompting a return to destination, which then deteriorated further and significantly
It's hard for me to think of a scenario with multiple serious illnesses where you would press on back to LHR. Maybe the folks were stable and, as speculated, got significantly worse approaching top of descent.
As far as holding the baggage for 'testing', I wonder if that was really the case.
I was a pax on a flight that had an RTO a few years ago and the bags were not available until over 12 hours later. Unfortunately, I was already in another country by then. Sounds like the RTO aircraft was towed to the maintenance hangar with pax but not bags offloaded hoping to quickly fix the engine and attempt another departure. Then the flight cancelled when the problem turned out to be more than a compressor stall.
On the other hand, maybe the AA109 plane was quarantined until a norovirus outbreak could be ruled out.