Im writing mainly about people on tourist visas which it seems many members are applying and getting at present. I only mention as i think a lot of brits forget about how much healthcare might cost if we had to pay for it.
When my wife was in a A and E observation ward last thursday they had to use several differnt special drugs, a slaine solution drip and gave her a free presrption for 10 days worth of tablets. Plus a bed for a day and part of a night which my wife was worrying how we would pay for i laughed and said we back in the UK now. In theory a vistor would not pay for that but the example shows how costs would bulid up quickly, things which we brits take for granted.
If your here for six months and you get run over you may need after care im sure they will fix you up, but if you need a bed/aftercare for a few weeks i bet the suits would soon turn up. That or you would need to go private or return home wasting the visa.
Normal UK travel insurance would be no good as the person would not be a uk resident, they would need one from their home country or one bought in the uk for non residents which im sure would cost far more. These would pay out like our travel insurance would if we were abroad.