Sadly, it was more or less legislated out of business in the late 80's. We were very restricted as to what we could spray, and with very negative public opinion, the industry eventually all but ceased.
One of our biggest earners was a product called Spodnam DC, nothing more than a basic 'glue' that was sprayed onto oilseed rape to stop the pods from shattering when harvested. It was a totally harmless product, but the flak we used to get from various locals was unbelieveable. One person claimed that his 14 year old Ford Capri had its vynyl roof peel off because of the dangerous chemicals that were being applied by air. It had nothing to do with the farmer down the road on his tractor spraying sulphuric acid onto his potatoes to burn the top of the plant off, in 15kt winds directly onto the housing estate the previous day. That must have been safe.
We were in excess of a mile away, and downwind, but it must have been our fault.
I'm surprised that aerial top dressing doesn't still go on though. That was a very useful task that kept us busy when nothing else had any chance of getting onto the field due to waterlogging.
As LowNslow says, the expense also contributed to the demise. back in the late 80's we were spraying 35 acres to a load, and charging £3/acre. I guess that we avaeraged about 300 acres each day, mainly working only dawn and dusk due to wind restrictions, but on occasion, 1000 acres a day was easily acheived.
I understand that work is still done on behalf of the Forestry Commision in Wales and Scotland, and heather/bracken control on the Scottish moors. I saw a Jet Ranger working up there last summer, but he was miles from his landing site, so I couldn't go visit them. Wonderful memories.