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!