I think that the prices are based on:
The fruit colour. Using the rainbow colours ROYGBIV = Red, Orange, Yellow, Green, Blue, Indigo, Violet we get Red (strawberries) = 7, Orange (satsuma) = 6.5, Yellow (lemons) = 6, etc. The blueberries are actually more like deep purple or indigo, which explains the gap in the values, the missing £5. That would be the value of a proper blue fruit. So, assuming that the bananas are yellow, then their price is £6.
A crazy guess on the 'milk' price. I remember this experiment in school - on a circle you have the 7 colours of the rainbow. The circle is then rotated with high speed, visually 'mixing' the colours into something that should be white (never pure white though, because the original colours are not perfect). So the 'milk' colour price should be the average (£5.5) or the sum of the 7 colours (£38.5).