When I lived in Sussex on a farm in the early 50's, hedgerow clearance was occurring. The first combine arrived on the farm after these hedgerow removals. Nothing new in making the fields size suit the machinery. Soil erosion is a fact of life in the UK especially in eastern England, yet farmland in norfolk still makes up to £9000 acre.
No sane person will pay that sort of money for sterile land so you are obviously exaggerating as usual. The underlying geology and superficials dictate the value of the ground and Norfolk contains the Brecklands and Broads that are largely subject to EU conservation directives. Cliff erosion has nothing to do with agriculture. it has been a feature of life on that coastline since Dogger was breached thousands of years ago.
"Without modern (ie Multinational) agriculture Britain would be forced to return to the land and produce its own food - which historically, it has always done exceedingly well rather than shipping it in from abroad"
In reality it has not done exceedingly well since significant wheat imports occurred around 1820. You seem to think there was some sort of golden age when jam butties grew on trees. Try feeding today's UK population with peasant techniques and see how far you get.
Internal drainage boards encompass nearly 10% of England's farmland and include 50% of all generating capacity and the UK's largest port.
Decisions are made on the basis of maximum benefit and cost. This also applies to sea defences. Not everyone can expect to be protected.
If you buy a cliff edge house you must be particularly brain dead to assume cliff failure cannot ever occur. Similarly for living in low lying areas and assuming floods on impact others.
As far as job opportunities are concerned Norman Tebbit phrased it rather well. He said "get on yer Bike". In my case I got on a jet(frequently) Away continuously for 2.5 years on my first job in SE Asia, and for 6 months in Borneo on my last job.
   You seem to constantly overlook one vital fact: Farming is a business and needs to turn a profit to survive.Wearing rose tinted specs and hugging the nearest tree will not change that harsh reality.
It is also a fact of life that holdings tend to increase in size over time to benefit from economies of scale, otherwise they go under.
If you lived on a farm, instead of a bog, you might obtain some slight glimmer of understanding. Only in socialist magic blunder land does the magic money tree exist.