I had the same thought, but of course the operator is not obliged to undertake the circuit work at OW, and 5 mins cross country will offer you Duxford, or Henlow, or Cranfield, or Fowlmere….. and so on.
Furthermore, the nature of the ‘course’ may be that ‘Pilot under training’ is not actually handling the aeroplane close to the ground, so the fact that the field is not massive is not a factor.
There is a difference between kosher flight training, and the ‘Spitfire Experience’.
I would have thought OW was plenty long enough, not sure how user freindly it would be however??