Who knows? Perhaps in the UK, they replace Charles Martinet's infamous "it's-a me, Mario!" clip with a different one?
Anyway, neither way is proper nor improper. The only way to find out is to head over to Italy and ask someone there.
I always use the "ah" sound, though. However, to me it makes sense that English people would pronounce it "ah" and American pronounce it "aa"... odd.
Anyway, it only makes sense that Japanese people pronounce it as "ah", because their language is extremely simple. Each vowel makes only one sound, ex:
A = "ah"
I = "ee"
U = "oo"
E = "eh"
O = "oh"
That should also help for the pronounciation of Lakitu.
I have one last question for "Matt"... have you ever played Super Smash Bros. or Super Mario 64? If you have, do they change the pronounciation for 'Mario' in the UK version?