if, during the interview, you just get lost with your english and start to have a really bad pronuciation and making basic errors on gramar?
That's what I'm afraid of, next tuesday...

Try to use the simplest structures and vocabulary. It's the first thing I do whenever I have to do an interview, a presentation etc. in a foreign language (for example English which as you have noticed is not my mother tongue). Ok, it's amazing to see that someone can use complex structures and rich vocabulary in his second language, but if you're not really sure it can be a great risk. Plus I think that English-speaking people (as opposed to speakers of other languages) are well used to hear foreign people speaking their language, so you won't probably be the worst English speaker they met.

I disagree with MistaG's step 2 (to a point) - if you feel you can expand your answer, then do so, even at the risk of using slightly wrong words or possibility not finding the English word you want (your interviewers are likely to help you out). It is absolutely crucial that you come across as enthusiastic - your potential supervisors will be less concerned about slight deficiencies in your language skills as they will very quickly improve when working in the UK. However, if they think you aren't excited by the project and keen to work with them, they are unlikely to offer you the position.

Try to be yourself, be polite and above all, let your enthusiasm for the project be evident. Good luck!