If UK farming no longer had support post brexit then Agriculture would shrink dramatically.Farming like any other business needs to keep in the black to survive. The EU has featherbedded farmers for decades (entitlements of tax payers money can be bought and sold). In or out of the EU subsidies cannot be eradicated without dire consequence.
And for our resident "well educated scientist", greening has been an essential part of farm payments for a considerable time.
I am surprised you do not acknowledge this, being an expert like!
" Farmers are going to have to do environmentally nice things, as yet unspecified, and open the land up, etc. (sure, let the public trample all over their fields!), before they'll get any dough"

Environmental measures were specified many years ago and getting more pervasive each year. Let us all have a few links to support your preposterous statement!


https://www.gov.uk/guidance/bps-2018
https://ec.europa.eu/agriculture/sites/agriculture/files/leaflet_en.pdf

https://www.soilassociation.org/blogs/2017/february/payment-windows-for-organic-farmers-opening-across-the-uk/