You're right - travel insurance is much more common in the UK, and a British person going out of the UK is regarded as almost irresponsible to not have cover. However, I'm well aware that visitors to the UK from the USA only rarely take out a policy. As far as I can see,here are the main reasons:
a) British residents are covered in the UK for medical bills on the National Health, which does NOT provide cover if they fall ill in (say) the USA, whereas Americans tend to have private health insurance which I've been assured by some of them gives full health cover no matter where in the world they happened to be. [Footnote - take an E111 to Germany to get what reciprocal cover that there is ...]
b) A great deal of American travel is domestic, whereas here a lot of it is international. Having insurance in place on international travel in Europe provides a degree of extra cover in relation to the legal systems of other countries (for example, bail bonds for Spain) which aren't necessary for domestic travel. And it's much more expensive to re-patriate acrosss international borders too.
c) The USA is a highly litigatious system compares to the UK. It's good practise to have personal liability and legal advise cover when going there as a Brit.
d) Many British people go on holiday on a package tour costing a few hundred pounds. Should they have to fly back early (e.g. due to a family problem at home) it can cost more than the cost of the holiday just for the return flight.
I'm not best placed to advise on travel insurance for a single trip as I travel enough to have an annual policy - I pay 165 pounds for the year for 3 or us with up to 183 days cover per year, with a maximum trip length of 91 days. I could probably save on that. Key covers of such a policy include
Medical Emergency and Repatriation
Personal Accident
Cancellation and Curtailment
Personal Baggage
Personal Liability
Legal advise and Expenses
Top up liability
Winter sports (up to 17 days)
and a few other more minor things such as missed flight and travel delay.
