Thanks for the information, it looks like the 1099 route is pretty straightforward for dealing with interest income. I am a UK resident by the way.
What about reporting, would it be likely to count as a foreign financial account (of a sort), and be put on the FBAR, which I have to do anyway?
At first reflection I don't think a p2p investment would be any of the harder ones to deal with like a foreign trust (so no 3520), or a PFIC (nothing to do with owning shares).