Hello,
I had my interview today and was approved. Thought I'd share my experience. I will do a full review in the reviews section for the London embassy but, in short, it was a smooth process. Took about an hour in total but most of that was sitting down waiting. I only needed to show my original civil docs and wasn't asked to produce any other documents. I was asked one question - how did we meet - and that was it.
One tip is when you get there you can skip all the queues and go straight to the desk outside and tell them you have a ds-260 for immigrant visa. Turn up there 30 mins before your scheduled appointment. You can also bypass all other queues after security. Queues are for non-immigrant visas.