Hi again, L!
Regarding population dynamics forward from the present. The first thing to say very emphatically is that I, along with everyone else, am speculating. No-one at all has any ‘totally reliable’ cast-iron factual certainties. This goes particularly for ‘Deagel’ (not Ed. Deagle who was a real, fairly unremarkable technocrat in the US power structure).
The best brief on Deagel that I can offer comes from the good James Corbett, here:
I recommend highly that you listen right through to the end, since James has some very pertinent comments there.
I’m based firmly in the JMGreer/Jim Kunstler camp regarding what happens to human society and human population forward from now. And like them, I expect the process to take a century or so, rather than being a sudden, Mad Max, mass-depopulation apocalypse. Sure, there are fantasists amongst the eugencally-minded gics who’d like to do such things, but like all the rest of us, they’re simply not up to the job; though of course they like to stroke themselves with the delusion that because they’re so much cleverer and generally superior to the rest of us, they could. Hah!
My hunch is that a lot of the second half of our current population-overshoot episode will be - quite possibly - barely noticeable beyond repeated year-by-year statistics showing a steady, slow decline.
Sure, there are likely to be passing, local catastrophes such as drought-induced famines and such. But the overall process is likely to be as unobtrusive as the first half, the growth period now coming, or come, to its end. This has happened over a couple of centuries to bring us to the zenith where we are now.
As John Michael points out acerbically in his writings, there’s a long, long history of Chicken Little alarmist predictions of imminent global catastrophe, often with specific dates given; and every one of them so far has proven to be moonshine. I know there are alarmist calls about imminent nuclear war right now. But, you know: work to oppose such lunacy any way you can, and - wait and see! And meanwhile, don’t panic and don’t indulge in despair.
A very high quality survey of our current situation is given in this conversation between Peak-Oil man Nate Hagens and Professor of systems-ecology William Rees, of ‘ecological footprint analysis’ fame. I posted a link to this conversation here a week or so back, to little comment. Here’s the link again:
Sorry that James C takes half an hour on his post, whilst - even worse!!! - Nate and Bill go on for two hours (oh how can a poor put-upon Pampered Twenty Percenter cope with such loads!!! 
)
But if people will take the time, the deep richness of savvy analysis and sound information is right there for the taking. I reckon these three, together with John Greer and Jim Kunstler, give as savvy and well-informed a prognosis as anyone whom I know.
To sum up: I suspect - cautious verb, notice! - that sometime in the next five centuries Britain will again have a low population, possibly within its natural ecological carrying capacity, though what the ever-cycling climate will be like then isn’t clear: possibly another ‘Little Ice Age’, with the Thames freezing over again in Winter; or possibly another ‘Mediaeval Warm Period’, when the Norse were able to grow wheat and barley in Greenland, which isn’t possible now, and they found grapes growing in Newfoundland! Hence ‘Vinland the Good’. We (in our contemporaneous re-incarnations in this upcoming future time 
) will see.
I hope steadfastly to see Yr Ail Coed Mawr - The Second Great Wood - re-clad Britain - and Eire - in that time. All my beloved trees back again, in their billions!
Regarding food-growing:
Start with taters! They’re easy and pretty reliable. Rewarding for a beginner. I’ve been meaning for some time to post a photo-account of my efforts last year, growing potatoes in turf/mulch. Sorry to be so slow. I can only just about keep going with my basic household chores to keep independent living possible, just now. If this heart-valve replacement works out, they seem to think I’ll have better energy again. I’ll post that account, with pics, as soon as I can.
But anyway, here in brief is what I recommend:
For low-work, easy growing, stick emphatically with no-till gardening. DO NOT disturb the soil. Every time you do, you inflict a terrible global catastrophe on the little souls making up the living soil community; all the small creatures from worms on down in size, who are the very essence of soil fertility. After each act of tillage vandalism, they have to do what the poor sods on the Turkish/Syrian border are faced with right now: haul the survivors out of the catastrophe and begin the painful task of re-building soil fertility right from the ground up, all over a-feckin’-gain! Don’t make their work harder by inflicting the tillage catastrophe, and they’ll reward you richly.
Also, don’t compact the soil! Stick to walking on narrow designated pathways through your growing beds, and stay off the growing areas.
If you can lay hands on a convenient patch of ground, proceed like this:
If it’s already turf, and if it hasn’t been tilled for several years, that’s a good start. It will already have the fertility conferred naturally by fallowing.
If it’s turfed over already, even with rough pioneer first-growth volunteer plants, you have two choices, depending on what you find possible: clip the growth down short, leave everything where it falls, then begin planting your spud chits into this ground. If you can lay hands on just a modest amount of mulching material, plant your chits directly on the soil surface (no need to dig down beyond just loosening the soil surface a bit under the chits); pour a mugful or two of compost over them, then make a scarf of mulch around them, to suppress volunteers, but with no covering actually over the chit itself, so that its sprouting shoots can see the light and go for it.
As growth gets going, go around each plant and clip back any volunteer who may appear. Perhaps every ten days in the height of the growing season. No need to be too fanatical about the volunteers; just keep them down a bit, so that they don’t swamp the growing haulm. Also add more mulch to the scarf periodically, as you can lay hands on it. Ruth Stout’s motto was that you can never have too much mulch, and more is always better. Jim Kovaleski, in his gurerrilla-gardening exploits, grows veggies in ground specifically mulched only with scythed grass: ‘grass-fed vegetables’. Almost any biodegradable material will serve (though avoid privet or rhododendron clippings. They have rival-plant killer phytochemicals in their foliage.)
If you can lay hands on a large enough supply, cardboard laid flat on the turf, then covered with mulch, is both an excellent volunteer-suppressor AND soil food. Cut cross-slits in the cardboard to plant your chits. Then mulch-scarf as before.
Everything gets pulled down and eaten by the worms, over time, and turned by them into their plant super-food: worm-castings.
Depending on the droughtiness of the season, you may have to water a bit. This is a good opportunity to add quick plant food to the soil. Buy liquid plant-food concentrate if that’s all that’s available. What I do is: when my local comfrey plants have grown to good bush size, I cut a few off at the ground (don’t worry; they’ll grow right back again from the stump; they’re tough as hell), stick the whole plant as is into a tub and fill with water. After about five days, you’ll have a brown, farmyard-smelling tea which is excellent plant food. As it ages, and gets deeper brown, water down at about one part tea to ten parts plain water. Water directly on each potato plant. They’ll let you know when they’re thirsty by starting to droop a bit.
My soil here is clay too, but growing in long-fallowed, never-tilled top-soil works fine. I got about sixty kilos of spuds from two kilos of chits last year. Still eating the last of them, stored in my cool, dark boat engine room!
Do an internet search for Ruth Stout’s low-work mulch-gardening; and visit Pete Kanaris/Jim Kovaleski’s YT videos for much more on how Jim does it.
Good green fingers to you!!