While pointing lasers at aircraft is a bad idea, this situation needs some further explanation.
As i understand it, they were hit by a laser at low altitude when departing from london, and proceeded to fly for an hour or so before declaring a PAN, after which they returned to london, not making an emergency landing at the nearest available airport.
This would be fairly odd - if one of the pilots is really incapacitated (as in suffered a stroke, heart attack or something like that) procedure would be for the remaining pilot to get the aircraft landed as soon as possible. In this case that would have been an emergency descent in dublin, certainly not a return to the specific airport in london (the city has 4 or 5) you departed from.
I have no idea if any injury has been proven, but it seems like they might just as well have completed this flight - seeking further medical consultation for a non-acute problem at the other end.
If one of the pilots was actually blinded on take-off that would be reason to land immediately on any compatible runway achievable. Given they happily flew on for an hour and then declared a pan, i'd say this is something they did to gain media attention.
In case of an urgent medical emergency one should land at the fastest airport in reach (which would have been dublin looking at the flight path), not take the time to return to the airport where the flight departed from. This sounds like one of the pilots noticed something odd in his vision an hour or so into the flight, wanting medical attention for that. That's a perfectly valid reason to land in a place where medical care is available, which would have been dublin, not london, in this case.