There was quite a stink about that when I was farming. Not involving me, I never used to go any further in the tractor than the other farm, which was about 5 miles away.
But at some stage it became known to HMRC that SEF were using tractors running on red diesel to move loads of cauli to their packhouse at Leedstown from fields all over Cornwall. HMRC asked them to stop, to which SEF came back that they couldn't. because it was simply not practical to use white diesel on the roads, and red in the fields, where the tractors spent much of their time.
They also pointed out that of they had to bear the extra cost of white diesel, the price of caulis would have to go up.
So for the time being, HMRC had to back down.
I imagine it is now felt that the growers have had enough time to make the necessary changes to procedure, so that situation is now changing.