Which area of London has the cheapest roast dinners?

The average price in South London is £17.32 (74 reviews)
The average price in East London is £18.14 (41 reviews)
The average price in West London is £18.40 (38 reviews)
The average price in North London is £18.77 (65 reviews)
The average price in Central London is £21.01 (67 reviews)