G-INFO says G-LUSH is a PIPER PA-28-151.
I doubt it would outclimb a Pitts, or whether the Pitts caused them any delay.
Its a bit like when someone in a Porsche uses the outside lane at traffic lights to out accelerate to get ahead of a learner driver. Pi$$es you off but....
I think Leicester is just A/G and maybe the Pitts just said "XX departing yy" and you missed it. He probably didn't go through the whole" Leicester this is G-XXXX requesting radio check for a local flight.... etc etc."
I doubt there was any safety issue