I don't understand this.
London Control will not even provide a service to anybody except on an IFR (airways) flight plan. For this an IR is required, so it's not their business to doubt pilot privileges.
I can only suggest the possibility, and please don't take this the wrong way, that the pilot did not sound very confident. In this type of flying, the workload isn't usually high but they expect you to be on the ball like an airline pilot who does that leg a few times a day and knows all the airway intersection names as well as your PPL instructor knows all the sheep within a few miles of his airfield. If your radio work is less than confident and decisive, ATC will get rattled because they are trying to separate you from commercial traffic. They don't have the patience of a monk of London Info, or Farnborough Radar, for example.
Same reason that a lot of people get refused VFR transits of Luton, Gatwick, etc, etc.