I think your guy should've bought a more efficient tractor, TGG!!
In this particular case, he has to pay 2 Polish guys £5 an hour and supply a van to drive an 80 mile round trip. Plus ferry crossing from the Isle of Wight which I think is pretty cheap for season ticket holders? The day itself costs £100. Obviously there's growing & harvesting costs too, but...
say he sells 400 punnets, that's £1,400 minus £100, £1,300 should just about pay for the growing!
I don't know where you get the tax figure from? There's no VAT on "essential" food and you pay tax on your profit, not your takings.
So I think this particular farmer is doing rather well if he does one of these a week. Plus he's got his garlic which he earns a fortune from.