Hi
I just called up Insure&Go (0800 781 7241) to ask if they do one way insurance and they do. It's basically same t&c and insured amounts as for single trip insurance, but it costs a little bit more.
You get 5 days cover in case you get delayed etc and the policy ends 24 hours after you clear immigration.
For me, my hubby and our 7 week old daughter, it would be £41.89 on the silver and £52.34 on the gold policy. The normal single trips would be £39.51 and £49.34 to cover us, so it's not that much more.
They said that most insurance companies will cover you if you're not returning, but that you need to contact them to arrange the policy and confirm that you will not be coming back (anytime soon!) when you take the policy out. I guess the cost difference would be similar as it is for Insure&Go...?
Anyway, it might be worthwhile contacting your normal travel insurer to find out if they will cover you for a one way trip.
PS - reason I was calling them in the first place was to check that they would cover my daughter seeing as she's so tiny!

Apparently, the majority of travel insurers cover kiddies from 0-18 for FREE on the adult policy. There might be a limit to the number of kids travelling per adult and restrictions on them travelling without an adult, plus the level of cover will be lower for a child, but they will be covered.