Bioethanol and biodiesel don't make sense
Having had a go at making biodiesel and looked at the economics of America's cack handed bioethanol production, I think that current approaches to biofuels are flawed:
Brazil has a successful bioethanol industry because they grow sugar cane in a subtropical environment, it doesn't require a lot of fertilizer, and it produces a high ethanol yield because you don't have to fiddle with it to get enough sugar out to make any ethanol at all. Corn and wheat as feedstocks, by contrast are pretty poor for giving a decent amount of sugar, requiring a malting process which uses lots of water and heat, or chemical treatment with substances derived from petrochemicals. To distill ethanol to sufficient purity to be a fuel requires vast amounts of heat. In sum, I think that the total energy return from the manufacture of bioethanol is close to zero.
Biodiesel is slightly better, in that getting vegetable oil out of feedstock can be done by cold pressing or with minimal heat. However the biodiesel process requires heat to be applied for some considerable time, and the use of caustic substances such as sodium hydroxide (caustic soda in the UK, lye in the US) to catalyze the process, and an alcohol such as methanol (produced from petrochemicals) or ethanol (see ethanol issues above) , to make it.
Converting a diesel vehicle to run on pure vegetable oil in the UK using a kit currently costs around £3000, but to build diesel engines from scratch to run on vegetable oil would probably add negligible cost to the price of a diesel engine, which was after all originally developed to run on vegetable oil. Vegetable oil, out of the above three options (bio ethanol, bio diesel, veg oil) requires the least energy and the least chemicals to produce. However, veg oil is also a foodstuff, which means that government would find it hard to centralize and hard to tax, and there are no tax breaks for veg oil as a fuel, so nobody is going to bother to build a plant to produce it.
If either UK party really believed in greening the fuel supply, they'd probably be pushing straight veg oil.
There is also research currently on oil producing algae, which would solve the problem of finding enough land to grow enough oil crops to provide fuel as well as food on a large scale, because it could be grown on the sea.