[Someone]

What feeds the grass? Chances are it partly is imported fertilizer (https://tradingeconomics.com/united-kingdom/imports-of-ferti...), and that the nitrogen in the fertilizer is partly washed out into the environment (https://www.wwf.org.uk/press-release/government-watchdog-fai...)

Also, chances are the livestock partly is fed from imported food, for example in winter.

[dflock]

The UK climate is, currently, very good at growing grass. You can continuously graze cattle all year round in most of the country, as long as you rotate them between fields every few days. If you have enough space, by the time they get back round, the grass is back. The cow shit fertilizes the grass, and silage made on the farm does a lot of the winter feed top up. As long as the density isn't too high, it can be pretty low input.