> They didn't have much runway left at 00:15 and were far slower than 158 at that point

You're correct. When they first annunciate G-A, they've missed the chance. (They almost certainly had enough airspeed to G-A even after touching down, though.)

My question is whether throttling up for G-A at 00:15 is what caused the overshoot to extend out of the airport (versus if they'd committed to braking). This will come down to runway conditions, braking performance and the throttle responsse of their engines.