It wouldn't have made any difference to this case, they were lured into a location and ambushed with grenades, even had they been armed they'd still have been killed.
Given that Britain's supposedly specialist trained firearms officers have a rather poor record when it comes to shooting the right people and not panicking in stressful situations the prospect of giving all police officers firearms is frankly terrifying.
As for the death penalty, giving the government the right to execute people is not the behaviour of any sane populace.