Why does this Strikemaster have a textured leading edge?
The leading edges are roughened to improve the stall characteristics. Typically vortex generators are used for this purpose nowadays.


  The 5 and the 5a were used by the RAF for pilot training and the nose strakes were fitted to improve the spinning characteristics - the leading edges of the wings had a roughened paint finish to improve the stall characteristics. The 5s with the tip tanks (I'm not sure if 5b is an official or 'unofficial' designation) were used by the RAF for nav training and didn't have the nose strakes (not sure about the roughened wing leading edges) I don't know if it was because the tip tanks themselves improved spin handling or if it was because they weren't flown by studes for spin training.


(source: User "Fritag" at www.britmodeller.com)


  The rough grey coating on the wing of the aircraft was applied in order to break up the smooth airflow and give an early indication of the onset of a stall as the T5's original clean wing design gave the pilot little prior warning.


source: all-aero.com / RAF Museum


  A great deal of work has been put into stalling, spinning and
  stall-warning over the years of Jet Provost and Strikemaster
  development. This has led to all sorts of devices, like
  inverted-aerofoil tailplane, fences under intakes, strakes round the
  nose, roughened outboard leading-edges and little "eyebrow slats" at
  the wing roots. Of all these devices, the Strikemaster requires only
  the "eyebrow slats" to give it that precious smooth spin, a tidy g
  break in the stall and a distinct stall warning.


source: Flight International 12 October 1972 500-501


  At first it was forecast that the T.5 would take the place of the T.4
  on certain roles such as high altitude training. Unfortunately the
  majority of T.4s had to be taken out of RAF service much earlier than
  anticipated so the T.5 had to take on more responsibilities. As a
  result, the RAF decided to update its fleet of Jet Provosts with new
  avionics suites, spin strakes, and roughened leading wing edges in
  1973.


source: Jet provost heaven