When traveling it is always sensible to have some sort of travel insurance and medical insurance. However virtually all standard policies exclude mountaineering over 7000m so for 8000m expeditions insurance is a lot more difficult to find.
It is worth understanding that in Tibet there are no golden hour rescues; no helicopter rescue is available. Any evacuation is normally carried out by our sherpa team with the help of some of our climbers to get you off the mountain, and then a yak or horse if you cannot walk to base camp. From there you drive back to Kathmandu where a western-staffed clinic can assist you. In most cases the idea is to get you back to your home country as quickly as possible.
It is sensible to be covered for at least part of your expedition so do consider taking out travel insurance, however ensure it will not be invalidated by your climbing. See our trekking and trekking peak insurance discussion.
Read Mark Horrell's blog post for a (British) itinerate commercial climber's opinions.