OP coooool so what we'll do is import produce? So you see real, fundamental problems with British farming - and want to export these inconveniences to some poor, faraway country so you don't have to deal with any conscience pangs? How very privileged of you!
This is my take as a massive leftie, seeing as you're probably George Monbiot.... It is genuinely meant to be helpful!
Cutting back on farming in Britain will lead to lower food standards. Lower health and safety standards and lower production standards (you want US meat pumped full of stuff? With Trump dictating what can go in it?). Research food standards and see in the UK why the government is so able to dictate to farmers what to use in their practises - there's too little research and farmers hate it. Eg sheep dip, cow vacs and pesticides - a generation ago they were told what to use, without proper testing. Exposure led to many suffering side effects worse than some industrial accidents. Yet still research and accountability is low. Help keep the government accountable! To farmers and consumers!
It will lead to terrible animal treatment. If the subsidies go - fine by me, I can afford it. But prices for British produce will go up for the consumer - including for meat and dairy. The cheapest markets are non-EU. And those closest to home around Europe are not held to our standards. Not raising the animals, handling them or slaughtering them. Brits won't buy EU produce as it'll be as pricey as British produce, knowing they can keep prices up for people to buy decently made produce. So more animals will go through the tougher systems. Either risky near-Europe farms, or factory farming in the US. An issue you could instead help with- improve UK slaughterhouse standards. Why do we need only a few mega ones not close to farms? A lot of the smaller, local ones were much better for the animals, causing less distress in transport. Accountability is no more improved - why? Investigate and help!
Perhaps the most sensitive issue is this: you're unwittingly massively screwing over a foreign workforce. Imports means screwing over workers' rights - let's make any crop workers who've come to Britain move back, they'll be on lower wages in their own countries and deny them the right to come to the UK while we're doing a hard Brexit? Maybe they'll have even fewer workers' rights there. Great idea for a mass exodus of workers you don't like the idea of? Here's a different issue to focus on: improving farming workers' rights in Britain. Go undercover with the crop workers and find out what issues they face - do they have proper access to or infrastructure (children in good schools etc)? How is their job security, can they report issues like whistleblowers can (without fear of reprisal)? Do some research and help!
Why are you focused on sheep in particular? This is my tip for you here: read Private Eye. Compare and contrast the subsidies sheep farmers get, with crop farmers who farm on a mega scale (with farm managers, thousands of very very profitable and valuable acres). Not all crop farmers are like this - but a really horrid trend is millionaire farmers (the daily mail owners!) getting subsidies for owning land and doing sweet FA with it. They hobby farm on a massive scale and take in cash for nothing, for crops that we don't need or that smaller farmers should be able to produce but can't compete with the efficiency. It's a post-capitalist horror, and pushes out any smaller party competition. On the other hand, sheep and dairy farmers just need to be able sell their produce for a decent price - have you seen the price fixing scandals by tesco et al? It's a case of being undercut to extremes, and then being told to be grateful for a subsidy when they'd rather just get the fair market price. Research how mega supermarkets are screwing smaller farms - these are the farms that are much more likely to operate at higher standards. Why? Because for smaller farmers it's personal and they take pride in their community. They have reputations to uphold.
Separately... I'm so confused and almost delighted by your comment about sheep on the hills! What did you think those hills looked like before farming? Woodland simply doesn't grow at that level! We had a year of not keeping up with our community garden and it was such a thicket, it wasn't even possible to walk through with all the brambles. So do you want national parks and rural areas to be re-wilded? Not accessible to those who benefit from visiting it now? Ok a lot of middle class lot go to the peaks and snowdonia etc but our local school benefits massively from visiting.
Or is this simply a case that your view of what we do with the countryside is inherently better? That you want it just how you like it? That it's for you to guard from the dumb masses? That you never have listened to other voices - whether we're from towns or countryside - and won't start now?
Mr Monbiot and his fan(s) - I suggest you listen outside your echo chamber. I do mean that kindly, we could do with decent activists on the issues that do matter