I know with Aviva (form Norwich Union) as long as you pay from a UK bank account and not living in an area deemed by the UN as an area of war or internal conflict youll be covered for life of the term.
If you can wait it out, and dont plan on keeping your UK bank accounts, youll be best to wait until you get back to the US to apply. In the UK, you need to be a resident of the country in order to apply. Similar rules may apply to US companies