I would guess that the # of 400m strips one could find would exceed the # of 800m strips by around 100 to 1.
This is simply a consequence of the way fields have been divided up by farmers over many years, and hedges have grown up on the lines and these are difficult to remove (overtly
)
Unfortunately, the types of planes that can operate safely from 400m grass are generally unusuable for going anywhere seriously, IFR. The very few exceptions would be a C182 (especially with the STOL kit), or some equally rare type of unpressurised turboprop.
I don't see a problem with setting up a good number of 300-400m strips around the UK, but - if done in response to GA airfields closing - this would also mean the GA scene shrinking to a level where nobody gets any utility value out of flying and it is just a "pop up for a little local on a nice day" kind of hobby. This would also result in the wholesale collapse of PPL flight training and we would get more or less what already exists in much of Europe.