I emailed my tame TRE at Flight Safety at Farnborough. (Now he's contacting someone in Chester)
"the minimum is related to the structural integ with regard to gust factor the lighter the aeroplane the stronger it has to be to meet the gust factor vs. speed."
As the Hawker is 50 years "young", it's probably an old UK or Hawker Siddeley definition that has been lost in the mists of time.
I hope this helps.
