Tell nothing but the truth. If you lie about anything and they suspect it, they will ask more (and more instrusive) questions and possibly send you to secondary screening in order to catch some dirt.
That said, since you already hold an entry clearance (visa), the questions will usually not be many, as an ECO has already assessed and pre-cleared you for entry.
So relax, as long as you answer the questions honestly and confidently you'll be fine (don't be afraid to tell the officer if there something you don't understand)