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, if you hold an entry clearance (visa), the questions will usually not be many, since the ECO already assessed and pre-cleared you for entry. However, if you do not hold one, be prepared for the possibility of a proper interview and above all, bring any documentation you can proving the purpose of you entering the UK