In my Birmingham interview it was one on one, we just spoke about jet turbine blades, melting points, examples where I've used maths to solve and engineering problem and where I have solved one without maths.
In my oxbridge practise interviews I was asked questions like "how does a wing work" and how combustion engines work. I was also asked to explain Young's modulus, necking and do some proof by induction.
An example question was "how much water do you have to drop through a turbine to generate enough electricity to boil a kettle" so I made up some numbers and rolled with it equating energies.
I plan to go casual to Cambridge. To Birmingham I just wore jeans and rugby top and ended up with an AAB offer at the end of the interview.
*Try to coax the interview into topics you know. If you know your interviewers research their papers. For example when I walked into my interview there was half a knee on the table (an implant), I just happened to know what it was and pointed it out so that led us on to material prosperities, a subject I'm quite comfortable with.