I guess the UK is rather different from the US. Or perhaps I should say the US is rather different from everywhere else.
I've never heard of people in the States getting "ski insurance". I do often read here or other ski forums about people, especially it seems from Commonwealth countries, wanting to buy "travel insurance" or "ski insurance". And a month ago when I was in London I saw all sorts of adverts on buses and in the tube about the very same thing. Seems like a very foreign concept here.
People in the States will sometimes buy "travel insurance" for high-cost trips that have large cancellation penalties, but usually the main goal there is to mitigate the risk of cancellation fees if they can't make the trip at all due to some emergency.
At the risk of sounding like a gaper, I'll admit that I bought that type of insurance once, my first time back skiing after a 10 year gap, when early season I booked a fully-prepaid air+hotel package to Whistler. I guess I felt like I had a lot of wasted costs paid in October if I couldn't make the trip in March. Never bothered with it before, nor after on any ski or other travel, international or otherwise.
I'll self-insure that risk nowadays, especially after seeing how many quite decent travel providers will bend rules if you have a valid medical issue and ask, rather than demand. A later trip to Whistler, where I did injure a knee and couldn't ski the last 4 days, and yet another trip there where a death-in-family caused a last-minute cancel, were handle very well by Intrawest refunding remaining pass days, and by Executive Inn refunding non-refundable deposits.
I guess the UK folks buy travel insurance that includes special riders for ski coverage, because of NHS coverage not covering foreign travel, or at least ski-specific travel health coverage. That makes sense - pop an ACL or do a tib-fib fracture and you're looking at serious financial pain along with the physical pain. If your UK-based (or similar from other countries with national-health type plans) insurance doesn't cover you out of country, then of course you'd need to buy something whilst here.
But now that you're in the US and have US-based employer-provided health insurance, you probably don't need anything special. It may not be great coverage, or it may be really good, but it's basically what you need. Depending on co-payments and coinsurance factors and deductables, a $20,000 ACL reconstruction and $5000 of PT post-surg rehab might cost you about $1000 out of pocket, or $5000, or closer to $10,000 (as my charming wife Lisamarie and I found out). However if you essentially self-insure that as an acceptable risk, knowing that the insurance is at least picking up the near-catastrophic parts of it, your employer-based insurance is probably all that you need.
As screwed-up as the US-based private-insurance hodgepodge of a health-care system is, and as expensive as it is to pay premiums, almost all health care plans do cover you wherever you are. Granted, if you're skiing glorious Kazahkstan, you may need to pay big rubles up front and probably would want medical evacuation, so a supplemental travel/ski-specific policy would make sense. But living in the States, with a US-based employer policy, you should be fine in the States, in Canada, in Western Europe, etc. In the US, the providers would probably bill the insurer and only ask up-front what they calculate they won't get due to co-pays and coinsurance and deductibles. Elsewhere, they'd probably want payment up front or in-full within 30 days and claiming it to insurance becomes your problem. But you'd still be covered, at least at the "out-of-network" level of your policy.
There's another thread here about "Would you ski without medical insurance" or some similar title. Interesting reading.
As far as somebody sueing you for hitting them on the hill. Not as likely as the bad press about the States being litigious makes out. Even so, if you have homeowner's insurance (if you're a house or condo owner) or renter's insurance (if you rent your home) you probably have some level of general liability insurance. That is most likely going to protect you up to that level for negligence. Check with your insurance agent, and as mentioned up-thread, consider supplementing that with an umbrella liability policy.
If you do have a big co-pay or deductible to pay to a hospital or doctor, at least you can make sure to use an awards card that gets frequent flyer miles so that once you're healed up, you've got miles in the scheme for a fun back-on-the-snow trip somewhere!
Note: IANAL, and my son dropped out of law school before finishing, so I know absolutely nothing for sure. Nor am I an insurance agent None of this is legal advice and none of this is given in any way as part of my being a part-time ski-instructor. Just another ski bum hanging around epic.
PS - can you tell that I just spent all day yesterday reading the fine print trying to decide between the three different health care policies my corporate employer is switching to for 2007?