Which area of London has the cheapest roast dinners?

The average price in South London is £17.82 (79 reviews)
The average price in East London is £18.14 (41 reviews)
The average price in North London is £19.06 (67 reviews)
The average price in West London is £19.10 (40 reviews)
The average price in Central London is £21.30 (70 reviews)