So I got some fluke injury the other day. I'm not a fan of hospitals, and am generally way more interested in just "walking it off" than actually heading to a medical professional, but eventually my friends coerced me into getting checked out.
So I got to the ER, filled out some very basic paper work, (my name, address, and emergency contact) and then sat down. They did not ask to see my insurance card or drivers license or anything. Since I did not have a life threatening issue, I than sat down and hung out for the next 2 hours or so. Eventually, I got called in, checked out, and was sent home. Just like I had assumed, everything was ok.
The biggest advantage about hospitals in London is that everything is free. The disadvantage is the wait time. I have had a few other friends have to visit the hospital in London so far and all of them have complained about the wait. So in the end, hospitals in London will provide similar care to the ones in the USA, with a longer wait, but absolutely no bill.