If you spend more time on the road than off it, this info may be useful .......
Up until recently I was fairly restricted to World Nomads for my travel insurance as they were the only company I was aware of that a) will insure you if you're not currently in the UK or b) if you are in the UK don't insist on you having been there for the previous 3 or 6 months in order to take out a policy. Whilst I was perfectly happy with World Nomads (although not having made a claim don't have reason not to be), their prices have crept up recently particularly now SE Asia is in the same risk category as the USA!
If you're in a similar situation you may want to check out Worldwide Travel Insurance www.worldwideinsure.com who were recommended to me by BIBA (British Insurance Brokers Association) - their prices seem pretty good for decent cover - they have different policies from a budget one that covers medical emergency stuff to more comprehensive ones. Most importantly they will insure you without being a stipulated time in the UK prior to commencement of the policy, will extend it whilst you're away and also have policies that you can take out if you've already left home. Of course the biggest test is how they respond in the event of a claim but I hope I never have to find out.
I'm not aware of what options residents of other countries have but looking at Worldwide Insures website they do also cover residents of USA, EU and "other countries" on similar terms so may be worth a look.