It's drummed into Brits visiting the USA that they need travel insurance ... primarily to cover medical costs if they have an accident or fall ill. So if you visit the UK and say you haven't arranged travel insurance, those same Brits (or rather the ones who have not looked into the situation) may indeed look at you and wonder if you've really thought things through and made any plans if something goes wrong. It looks like you HAVE thought things through, though. You say you're fully covered by your parents' health insurance (and unlike our NHS cover, that may extend worldwide), which is great ...
Be careful when you consider what the NHS provides for "free" ... I picked up the following link under the subject "Entitlement to Free NHS Hospital Treatment for Overseas Visitors":
http://www.careline.org.uk/Doc.asp?WSDOCID=377 which says
No free hospital services (except treatment at A&E outpatients). except for visitors from certain countries and tht list does NOT include the USA.
Some matters for you to consider to help you decide whether you need any extra insurance when coming to the UK.
* In the event of a major accident or illness, do you have cover for ongoing and perhaps long term treatment once the emergency aspect has been dealt with
* Do you have repatriation cover (alive or dead)
* Do you have (or want) other insurances such as cover for your personal things if they get stolen, or for extra costs if your flight is cancelled, your airline goes broke, or you have to rush back to the USA because a relative's ill there.
This whole insurance business could be hugely important for anyone who gets it wrong, so please excuse the longer answer and the repitition of things you might already know; I have tried to research this and make it more generally useful that just to answer the original question and - PLEASE folks - follow up with any personal experiences.
Just one further thought. The UK is (I think) unique in having a "free" health service but in reallity Brits know that they pay for it through their taxes. There has been a lot written in the papers about overseas visitors coming to the UK to take advantange of health care to which they have not contributed, and this has lead to some resentment. If you visit and say "I didn't get any insurance", people just might wonder if you've decided to take undue advantage of the British system.
