There are loads of threads about this ...... maybe a search would have helped?
Now that I'm here; if you use a tv (receiving aparatus) for watching 'live' tv, you need a license; whether or not you buy one, is up to you ...... the chances of being caught are very small if you refuse access to anyone without a warrant to search and say absolutely nothing.
There has yet to be ANY conviction using evidence from a "detector van", so ask yourself if they actually exsist in the way that the TV Licensing people would like you to believe.
Choice is yours, risk is yours .......... if you go for the risk, make sure that you inform all housemates etc about not speaking to or giving access to the so called enforcement officers, otherwise they will get you convicted.