Most cattle and sheep in the UK are farmed extensively - on pasture. Cattle over-wintered indoors are principally fed on silage, much of which is grown on land that is great for producing grass but not so good for producing crops.
You’re making many claims without sources, but for the sake of argument I’ll assume you’re correct.
This may be true of British grass fed beef etc but we’ve already discussed in this thread that every time someone eats a chicken sandwich from Tesco or a burger in a pub etc they have no idea where the meat is coming from, and a huge amount of it is from places like Thailand and the US where the animals are absolutely not grazing on pasture. I know that few self-respecting mumsnetters would EVER confess to eating meat that didn’t come from a local butcher who can tell them the very name of the cow before it died, but it’s obviously not actually the case that people are exclusively eating high-welfare, organic, British, grass-fed animals.
Much of the supplementary food they eat is produced from the wastes of human food production - for example, soya residues left after the oil has been extracted.
This is simply not true. 70% of soya is fed directly to livestock. Only 6% is ever used for human consumption. The remainder is used for oil. www.onegreenplanet.org/environment/why-tofu-consumption-is-not-responsible-for-soy-related-deforestation/
Some of it (lucerne, stubble turnips) is grown on arable soils to give them a break from cereals - lucerne is a nitrogen fixer, for example. Cows and sheep have this amazing capacity to turn stuff we can't eat (grass, maize stalks etc) into high-quality protein. And yes, some crops are grown for them on arable land (this is very true of chickens, though I suspect - I don't actually know - that a lot of what they are fed isn't fit for human consumption). But in terms of beef and cattle in the UK, grass and food wastes are the major part of their diet.
Again, no sources, and again this defence only holds up if you’re literally only ever consuming organic, grass-fed, british animals. Maybe that’s true of you personally and you don’t ever have a packaged meat sandwich from a supermarket, but if so you’re in a tiny minority. Most people in this country don’t know where their meat comes from, or the environmental havoc wreaked in its production. The enormous market for packaged sandwiches and meat-based convenience foods is testament to that.
The uncomfortable truth is that there is simply no metric by which you can judge the environmental impact of different diets in which meat consumption comes out on or near the top:
ourworldindata.org/environmental-impacts-of-food
ourworldindata.org/environmental-impacts-of-food#the-carbon-footprint-of-eu-diets-where-do-emissions-come-from
www.google.co.uk/amp/s/www.bbc.co.uk/news/amp/science-environment-46459714
www.bbc.co.uk/news/science-environment-49238749