I'm currently at Imperial doing an MRes, and I was interviewed and offered a place on a 4-year PhD program here, last month.
Try not to be nervous. My advice, go in with some really good questions to ask the panel at the end of the interview that would impress them, like "is there any opportunity to be involved in BSc lab demonstrations during the PhD?". It shows that you're keen and that you've prepared well. Also, come up with answers to a few basic questions, i.e. "so why do you want to do a PhD?", "so why imperial?" or "so what do you think sets you apart from other candidates?".
Also another thing, be confident, yet not arrogant. You've been short listed so you're obviously a worthy candidate! You just need to prove to them that you are the one for the position, and that you will do well!
Good luck!