What if London became a solitary nation?
Become octilavo vegetarian to better support the city.
Egg farms in parks and on roofs.
Dairy cows in parks.
Local fishing.
Rooftop farms for vegetation.
Bee keeping in building, on roofs and in parks.

What if, a black market for meat developed?
Only the super rich could afford the black market meat.

What if, disease due to diet went away?
Lower diabetes, heart disease and obesity.

What would happen to chain restaurants?
With no source to buy meat, the chains would most likely close, bringing back the local community heavy shopping. Maybe only the rich could afford chain food due to the expense of meat.