Where can I buy annual multi-trip travel insurance?

I think it will have to be a Japanese policy; I found that all the UK insurers set some kind of UK residency requirement, mostly being in the UK for six out of the last 12 months but the one I used merely asked for a permanent address in the UK and registration at a UK GP. As none of these will apply to you, it has to be Japanese insurance, which of course means that medical repatriation will be back to Japan.

note that mult trip policies set a limit on the length of each trip, usually 30 days.

Agree with Taggie get 90 day from UK - remember the most important thing is if you hire bike ( which I would never recommend - roads and bikes to dangerous ) is to read all the small print with a fine tooth pick otherwise what you might think you are covered for you are not. If you are going Jungle trekking then see your GP to get your jabs or malaria tablet's before you go. Also check to see what activities your insurance company will not cover you for like water rafting , Bungee , Sky Diving ,water sport's etc. Hopefully you will have a wonderful time in the Mountains in Nepal, wake up in a Hotel in Kathmandu , open the Window and see the Clouds below you and then enjoy Paradise on earth Thailand

Post Office.

AIU that was what i bought thru my travel agent, i bought it for 1 year, covers medical and travel problems. since i travel from japan, i traveled to the usa, south america etc. and i was covered for a year.