Crop spraying has just about died out in the UK because of the falling price of crops meant that faremers could not afford it like they could in the good old days of the late 70s and early 80s. Some aerial was still being done on spuds in Norfolk/Cambs as late as 1997/98 but now it is largely limited to a small amount of bracken control on the grouse moors.
Unfortunately, in the years since then the tree huggers have had a field day and more restrictions on chemical use have been introduced, requiring such things as buffer zones between the target crop and water-courses (how many fields are not bounded by a ditch?) etc. Previously some these rules only applied to aerial application, but increasingly Farmer Giles and his ground sprayer is being forced to comply with advance notification and other rules. Also, whenever a spray plane pitches up all the NIMBYs for miles around rush out to scream and shout and complain to the Police, H&S, Environment, Local Council, Fire Brigade, Citizen's Advice - you name it. The hassle factor when spraying anywhere other than on the moors is now very high.
There is one more killer, and that is the supermarkets. These days Mr Tesco and his mates control everything that Farmer Giles can do. They control the way the crops are grown, even down to the types of chemical that can be used and the growth stage of the crop at which they must be applied. If you do not comply with Tesco - or ASDA, or Sainsbury or M&S - requirements, they will not take your produce.
This attitude has been driven by the tree huggers, who demand that sprays do not kill bees and the likes and as a result a lot of new chemicals have been developed which kill bad bugs but not good bugs. There have been bee-friendly sprays for years, but new chemical clearance rules mean that a separate clearance has to be obtained for aerial application of a new chemical. Because likely sales of spray for aerial application will be low the maufacturers cannot afford to get the clearances. No aerial clearance - no spray.
The bottom line is that aerial spraying is perfectly legal and there is a range of chemicals which can be used, but there are few companies with a CAA Aerial Application Certificate and few farmers have been prepared to pay for it. However, wheat has moved from £65 to £160+ a ton, rapeseed similarly. Spuds have previously been sprayed from the air in large areas, they are a high-value crop and they too have increased in price. Perhaps the increase in prices will see a return of aerial in the UK, but I doubt it now that the supermarkets and the tree huggers have had time to get the upper hand.