If it is a medical diversion, surely Cork or Dublin would see the ill person seen on the ground more quickly. Or is fuel at LHR cheaper than Ireland? Seems odd to burn all that fuel returning to London and flying over several suitable diversion points. I see that Sigwx talks about a medical diversion rather than a medical emergency, but if it requires a re-route, surely on the ground is the best place to be?

Or was it a crew member or pilot? That needed a replacement. That makes sense.