The problem with "sell the land" is sell the land to whom, for what?
I've no doubt much can be sold out dairy production into something more immediately profitable, like housing, or rich people playing games of one sort or another.
And the farmer may do nicely from this.
But the country won't.
Unless we're currently in excess, we'll have to replace that milk production with imports.
We will lose food security, which is always an issue for the UK (hence rationing in wars). And if farmland is built on, that loss of food production is even harder to reverse.
What's more, the price of dairy products will become more closely dependent on the price of oil, as we fly milk in from across the globe.
As described above, there are always difficulties getting the balance right between guaranteed pricing, as with the Milk Marketing Board, and cartels destroying a nationally essential industry by underpaying.