Oxford interviews (and probably UCL too) will depend on the tutors. Which Oxford college are you interviewing at? I can at least tell you what the tutors are like.
In my interviews at Somerville college, I was required to choose a subject from a list given to me 10 mins before the interview and then talk about it when I got in. The interviewer then stopped me at various points to ask questions.
I had two interviews, I chose something like "electronegativity & bonding" for inorganic, which went very well, and "nucleophiles & electrophiles" for organic, which was ok, but by no means great.
Somerville college doesn't have a physical chemistry fellow, so I didn't have a physchem interview (thank god)
Also bear in mind that this was four years ago.
Rule one - RELAX. I know it's difficult.
Rule two - never give "I don't know" as an answer. You WILL be asked questions that you don't know the answer to, and you have to be comfortable with the idea that it WILL happen, otherwise you'll panic when it does - which causes you to break rule one. They are looking for people who can think on their feet and not give up straight away. Any reasonable stab at the answer is MUCH better than "I wasn't taught that" - because that is the the whole point of the question, they already know that you don't know the answer, they wan't to see how you cope with it.
Also, as an obvious side note, don't drink alcohol the day before your interview(s) - at least not heavily. You would be suprised how many people miss interviews, get barely any sleep or are hungover for their interview, because they were partying with their new friends - you could be called for additional interviews with little notice too.