If they are like other places they could asking anything - so don't bother planning for it. You can see that from the responses! It will usually be just a 'friendly chat', to make sure you aren't totally barmy. The grades are what count. If you get the grades, they will want to have been nice to you at interview so you will not be scared to go there!
If prof is in a cheeky mood, or bored, he might say 'integrate that!', while pointing at the blackboard to something way beyond A level standard.
Don't panic.
If you can't do it, and are fairly sure it's a tough problem, just smile, and say, 'I can't. I've applied here to learn about stuff like that...' Then, hopefully, he'll laugh and spend the rest of the time being extra nice...