The J (exchange) visa application requires that "health insurance" be purchased.
I've bought UK travel insurance (including medical claims etc) for the 6 month period. Is this different from "health insurance" and if so will I have to buy that too?
Travel insurance usually covers a range of problems that might happen while traveling including lost luggage, rental car towing, and tour cancellation fees, not just health insurance.
As to whether any type of insurance meets requirements, that can vary as there are hosting agency requirements and there are government requirements:
University or hosting agency requirements vary, but for example my university requires that J visa holders (Postdocs, visiting scholars, etc) have insurance plans that meet ACA (Obamacare) requirements. Like other forms of health insurance, this would cover you you if you became sick or injured and needed longer term medical care. Note that with some plans the deductible might be quite high and might not cover regular doctors office visits, so it might not be suitable for things such as the flu or maintenance of diabetes or high blood pressure and that sort.
Many travel plans in contrast only cover emergency health care (enough to get you stable enough to travel back home) and don’t meet ACA requirements. The insurance plan itself will tell you if it qualifies under the ACA. If there’s no mention, then it doesn’t. Still check to see if they cover routine health care needs, prescription medications, and minor injuries.
As to what the US government requires for the J visa, this site notes:
Either way, the US Department of State has minimum J1 health insurance requirements that your insurance plan must meet (which applies to J2 visa holders as well).
Medical Benefits of at least $100,000 per accident or illness Repatriation of Remains in the amount of $25,000 Expenses associated with the medical evacuation of the exchange visitor to his or her home country in the amount of $50,000 A deductible not to exceed $500 per accident or illness A policy underwritten by an insurance carrier with: an A.M. Best rating of ‘‘A-’’ or above; a McGraw Hill Financial/Standard & Poor’s Claims paying Ability rating of ‘‘A-’’ or above; a Weiss Research, Inc. rating of ‘‘B+’’ or above; a Fitch Ratings, Inc. rating of ‘‘A-’’ or above; a Moody’s Investor Services rating of ‘‘A3’’ or above;
The same site notes that government requirements change and its best to always double check you have the latest information.
A side note: get as much health insurance as you can, even if you’re young. One of our post docs had a medical emergency and is being billed $150,000+ for a cardiovascular incident. The insurance plan this person chose was a minimal plan with a $100,000 per incident cap so they are potentially liable for $50,000+ out of pocket.
American health care costs lack any rationality.
@RoboKaren has a good answer but I want to expand on it a bit.
Common uses of "Travelers Insurance" in the USA consist of many factors. A lot of times having nothing to do with health care.
I will try to add more as I think of them.
The medical component of travel insurance is designed to deal with the immediate aftermath of an accident or medical emergency and get you home. Once you get home, the travel insurance's job is done. Any further treatment would then be under the normal arrangements in your home country (in th UK that would mean the NHS).
Regular health insurance is designed to cover not only the immediate aftermath of an accident or medical emergency, but also the more long term impacts. There is also an element of socialism in it where, since the "affordable care act", insurers are required to provide cover for people with pre-existing conditions, even if individually those people would be a bad deal for the insurer.