Hi!
I also had to take an interview before being accepted. There is no test, they are mostly looking for your enthusiasm in learning the subject for which you have applied. They asked me some questions related to my personal statement, things written in my PS which I had to explain more broadly, they asked me why I want to study the chosen subject (Biology) and why I chose the UK (I'm also from outside the UK). Then a few questions apparently unrelated to the course I want to study, like my hobbies, what will I do besides studying, etc.
The interview is quite easy to pass, just be enthusiastic and confident.
They accepted me right after the interview, so before ending the interview they told me I was accepted.