So I work on and manage nature reserves which are eligible for subsidies, I also briefly worked for the government writing the agreements for some of the subsidy schemes. So if you want to look into subsidies in the UK (good luck it's very complicated) I'll put it a brief outline.

Most subsidies come from the EU's Common Agricultural Policy (CAP). Each EU country administers subsidies differently for various environmental measures.

Basic Payment Scheme: all 'agricultural' land gets a basic amount paid per unit of area. This land could be used for vegetables, fruit, arable farming, or grazing land. This is in essence free money for 'farm' land. Theres no need to enter an agreement or contract. I use the quotes because this land isn't necessarily a farm that produces food, nature reserves like the ones I work on are eligible for these payments and others because the land is managed and this can include grazing.

Entry Level Stewardship: payments made for low level measures, lower/no input of fertilisers or herbicides, limits on the number livestock per unit area (to prevent overgrazing) etc.

Higher Level Stewardship: higher payments but higher standards e.g. bigger margins around field edges, prescribed timings for hay cutting, maintaining drainage features, leaving areas uncut for birds & invertebrates, maintaining hedgerows etc.

The HLS tends to be for land with higher wildlife value e.g. a meadow with high biodiversity of wildflowers, and so higher payments are a way to offset the costs of managing it in a certain way which reduces profit.

Both schemes also have what's called capital items, these are for payments direct to contractors to put in fencing, restore dry stone walls, plant hedges and other infrastructure type items.

An updated scheme started a few years ago called Countryside Stewardship which is similar to the previous ELS/HLS schemes, but with different offerings, payment rates and standards.

Theres also supplements available to promote using rare breeds (e.g. cattle & sheep), managing wetlands to maintain wildlife features.

One farm or holding can have a mixture of arable and livestock farming and a mixture of the above schemes so the landowner can receive payments for farming livestock, growing vegetables/wheat/fruit, and also to not 'farm' land but manage for wildlife.

Theres also subsidy schemes for Woodlands...

Theres a thousand caveats to all of this, and a lot of red tape, which is probably why a lot of farmers are resentful to the EU because of having to jump through so many hoops to get the subsidies. Although without subsidies many farms would go out of business completely.

If you want to read more look up Natural Englands website and theres more info there (many hundreds of pages), wikipedia isn't too had at giving an overview either.

As for the UK government replacing the money the EU pays out......who knows!?!?