In my experience... Of baggage.
UK version of a 22" case will fit.
US version of a 22" case (which is overstuffed to the maximum with everything but the kitchen sink and expanded every which way it is possible) will not fit.
I was pleasantly surprised at how large the lockers were on the upper deck. It struck me as the lockers not being overly small, more as complainers having bags that are too big and in all reality probably should not be onboard the aircraft to begin with... But hey, thats just MY opinion...