Which borough of London has the cheapest roast dinners?

The average price in Richmond upon Thames is £16.69 (9 reviews)
The average price in Wandsworth is £16.75 (12 reviews)
The average price in Lewisham is £16.94 (8 reviews)
The average price in Merton is £17.58 (6 reviews)
The average price in Waltham Forest is £17.66 (5 reviews)
The average price in Kensington & Chelsea is £17.67 (13 reviews)
The average price in Lambeth is £17.69 (16 reviews)
The average price in Hackney is £17.77 (25 reviews)
The average price in City of London is £17.90 (11 reviews)
The average price in Camden is £18.86 (36 reviews)
The average price in Tower Hamlets is £19.06 (16 reviews)
The average price in Islington is £19.43 (21 reviews)
The average price in Southwark is £19.44 (22 reviews)
The average price in Haringey is £20.50 (5 reviews)
The average price in Hammersmith & Fulham is £20.58 (12 reviews)
The average price in City of Westminster is £21.88 (39 reviews)