The UK has a population density of over 600 per square mile. Oops... When the capital development of a country becomes a by-product of the activities of a casino, the job is likely to be ill-done. — John M. Keynes
While the rabbits here are plentiful over the summer, over the winter there's really nothing at all available for picking off a tree - unless you grow it, preserve it, or both.
In medieval times, 15-30 acres (6 to 12ha) were considered enough to support a family. Modern farming would be - at a guess - 5 to 10 times more productive.
So you could homestead successfully and possibly also be energy self-sufficient on around 3-5 acres, or perhaps an acre if you're a sustainable farming god.
But it would be organised seasonal growing and storing - foraging wouldn't be a big part of it.
Your self-sufficiency still depends on reasonable weather. If it's too wet or too dry or too stormy or too sunny, your crops fail and you starve.
The most useful thing civilisation does is manage storage and distribution of food to create a surplus and a safety net. (For most of the world.) The second most useful thing is management of infrastructure, including irrigation, drinking water, drainage and power distribution. The third is centralised access to expert services, including R&D/innovation, and medical care.
If everyone is a smallholder, all of those become more complicated and potentially less effective.
