Hello.
I'm just going to say that I know I am very lucky in the respect that I go to a private school and our Sixth Form only has 25 students a year. This means we get quite a bit of one-on-one help with university applications and interviews.
However, my Head of Sixth Form has always said to us that practising fluency of communication is very important, not necessarily practising the interview itself- you don't know what'll come up!
Learning to articulate your ideas clearly is helpful in any situation, and to practise that our teachers often make us elaborate after answering a question in class, or we arrange 'debate' sessions to aid students who will be facing interviews.
Maybe you could arrange sessions with your friends and make them ask you a question- then, don't just answer it. Elaborate! Practise keeping on topic, but expanding your argument and using details. Doing this with someone who doesn't do the same subjects would also be helpful- ask them to ask about bits they didn't understand, or that you didn't make clear enough.
The great thing is with this form of practise is that you can be anywhere- bus, car, train, or even walking to your next lesson.
Also, have fun with it ahaha.
I've applied to Lincoln College, right next to Exeter! I picked it because I liked the small year groups and I really liked the position of the college. I nearly applied to Exeter though!