I did a lot of research into this and came up with very little. I'd been living for a few years in Japan, and then had come to Britain on holiday for six month, and hadn't even thought about travel insurance. I didn't even qualify as an American resident, let alone a UK resident.
http://www.worldnomads.com/ was pricey, but in their FAQs said that you could purchase their policies if you'd already started your trip.
I ended up taking out a year-long expat policy with
http://www.sevencorners.com/, which didn't cover lost baggage, delayed flights, etc. but was better suited to my needs. They had a travel insurace as well, which I didn't qualify for.