Turning Salisbury Plain into farmland might add a couple of percent to the UK's self-sufficiency quota.
A combination of reduced meat production, re-farming of MOD land, amateur vegetable growing and lower consumption would very possibly cover everyone's basic needs in the UK.
It might not be very exciting, but not eating crap and getting more exercise might not be a bad outcome.
Of course this only works for the UK. Rest of World is just a little more complicated.
