According to Tony Blackman's book 'Vulcan Test Pilot' Roland (Roly) Falk practiced a few barrel rolls from Woodford on the 31st August 1955, possibly in XA889 (without warning the rest of the crew!). He then performed rolls at that year's Farnborough show, possibly in XA890, telling no-one except the show's commentator.
Of course it generated a lot of interest but Falk inevitably got a ticking off from the authorities and was forbidden to repeat it, even though he was able to show that there was positive g throughout the manoevre which was carried out only whilst the plane was climbing. He got his nickname from this episode.