Zopa replied to me but I don't think they specifically answered the question I asked which was about FATCA reporting:
"Anyone who's at least 18 years of age with a UK residency and UK current account can lend at Zopa without any restrictions, given they can provide evidence upon request. Reports of annual returns on all lenders are sent to HMRC."
you'd certainly need to pay tax on it - difficult question as to whether it would be classed as a PFIC or not, but there is definitely a risk that it could be classed as a PFIC when reading the definition of PFIC liaised below. Likely the best option would be to invest in the US where you would get the standard tax reports listed below
I don't have a bank account or any income in the US. I would never invest there as it would just create different problems with HMRC here in the UK.
Bank savings rates are so low now. For someone with a bit of savings it hard to put it to good use without incurring difficulties either with HMRC or with IRS. It's really frustrating.