Hi,
I'm not a US tax person, but my sister is, this was her response to the Roth IRA question:
"IRAs are only available to you as investments if you have earned income in the US. You have earned income in the UK that gets reported on your US return, but it then gets excluded because you pay tax on it in the UK. This rule keeps you from having to pay UK and US tax on the same income. However, it also means that you aren't eligible for IRAs at all b/c you don't have any US earned income."
{Disclaimer section} My circumstances probably aren't exactly the same as yours, so for a specific answer for your situation you should contact a professional.