Just wondering if it is OK to hit some wrong notes during DipABRSM exam. Would that fail u ? Or is it the overall effect is more important ?

You won't fail for a few wrong notes so long as the rest of the playing is good: even concert pianists expect wrong notes in every concert. If, however, there are a large number of wrong notes that begin to affect the performance the examiners may consider the playing not to be accurate and well-rehearsed enough to pass.