An interview slot appeared over the weekend for yesterday in London. Simple process, questions similar to those asked by a CBP officer on entering the US as normal. Got my approval email before I had made it back to Vauxhall station.

To anyone still hoping for an appointment in London, don't give up hope. I spoke to 6 people yesterday while waiting and half of them had only booked their appointment that morning so keep checking if you can get there at short notice. I'd even say it wouldnt matter if you were a bit late because they don't call you in by name, you kind of have to figure it out amongst the people waiting.