Depending on how loosely you want to define the term "car", I would think a later Reliant, either a Rialto or a Mk2 or 3 Robin would be about the cheapest thing on wheels imaginable. They do like 60mpg and appear to go for no more than a few hundred pounds a piece, and IIRC have all sorts of discounts on insurance and road tax. The bodies are fiberglass and the chassis are galvanized, so as their old ads said, you'll get sick of the car long before it ever wears out.
However, I understand there's a massive stigma associated with those things over in the UK, probably even worse than the one that BL and ex-BL products have, and of course, they're not even cars in the strictest sense of the word.