UK 9.34 million hectares agriculture area. [1] 6,470 l/ha ethanol in Brazil [2]
60,4 billion liter ethanol for all agricultural land in the UK. (Surely a big share of the land is not good for this usage but what ever.)
About 45 billion liter of gasoline and diesel used in UK per year.[3]
I mean, it is not totally off by orders of magnitude to be practical for a significant share of driving.
[1] https://en.wikipedia.org/wiki/Agriculture_in_the_United_King... [2] http://www.globalbioenergy.org/uploads/media/0907_Goldemberg... [3] https://www.racfoundation.org/data/volume-petrol-diesel-cons...