My dealings with government officials is never give them more information than they ask and always tell the truth. If you need to lie believe the lie until it is the truth ;).
If you are in transit, quite common for Heathrow, don't mention you have family in the UK unless they ask for family in the UK. If they do ask know where they live. If you have a tourist visa don't mention work you plan to or might be doing in the near future in the UK.
They are there to try and trip you up in a lie so they can question you further. You know how you can tell someone is lying by the way they go into too much detail on some aspects of their story but not others, don't be that person.
If you are in transit, quite common for Heathrow, don't mention you have family in the UK unless they ask for family in the UK. If they do ask know where they live. If you have a tourist visa don't mention work you plan to or might be doing in the near future in the UK.
They are there to try and trip you up in a lie so they can question you further. You know how you can tell someone is lying by the way they go into too much detail on some aspects of their story but not others, don't be that person.