Just a quick update on this subject.
I was in st thomas hospital (opposite the houses of parliment) today on business and as i walked past the pharmacy in the lambeth wing.
I saw that they had a sign up saying A and E care is free but you could be charged for any other care. Please contact the overseas unit (or something like that) for more details.
So im sure most local town hospital does not have such a unit but at least one in london does and anyone on a tourist visa should be aware.
Its not a subject people like to dwell on but if you are on a tourist visa and get knocked down it could be very expensive.