Which borough of London has the cheapest roast dinners?

The average price in Hounslow is £15.68 (5 reviews)
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 £17.28 (9 reviews)
The average price in Merton is £17.58 (6 reviews)
The average price in Hackney is £17.77 (25 reviews)
The average price in City of London is £18.53 (13 reviews)
The average price in Waltham Forest is £18.88 (6 reviews)
The average price in Camden is £19.35 (39 reviews)
The average price in Kensington & Chelsea is £19.61 (16 reviews)
The average price in Lambeth is £19.63 (20 reviews)
The average price in Tower Hamlets is £19.86 (19 reviews)
The average price in Southwark is £19.97 (26 reviews)
The average price in Islington is £20.42 (25 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 £22.22 (44 reviews)