DEFINITELY buy travel insurance from the US before you travel. Even if it is just care in an extreme event (hit by a bus) kind of thing.
If for some reason you do need medical attention - do get it! Just be sure to get a written bill and proof of payment. You will be charged 150% of the cost as you are not eligible for NHS. As Larabee said, if there is an outstanding bill of £500, it is an automatic visa refusal.