I read that he didn't fly through the Red Zone so just proved that there was no ash present which is what the VAAC model predicted anyway. The CAA have his radar tracks. Also he had no scientific equipment on board to measure anything so was relying on the mark 1 eyeball of his pilots. His post flight inspection was presumably limited to an engineer with a torch checking the aircraft for any signs of ash.
Whereas BA have a 747 fitted out with special filters that collect samples as they fly along. And I think they do a boroscope of the engines afterwards. This would take the guts of a day.