Which area of London has the cheapest roast dinners?

The average price in South London is £17.24 (72 reviews)
The average price in East London is £18.01 (38 reviews)
The average price in North London is £18.25 (59 reviews)
The average price in West London is £18.40 (38 reviews)
The average price in Central London is £20.52 (62 reviews)