No, you were lied to or more likely asleep at the raining.. Phone them up and read their website or email them, they all answer the same way.
YOU do NOT have to prove anything. YOU do not even have to let them in.
THEY have to prove you are watching TV. which is very hard to do. Just have a TV and a roof aerial does not cut it in a court of law.
Then I will beg to differ, the only way to prove it is for you to have a T.V and use it to watch a live broadcast and have no license and see what occurs.
I can only go off what I was told and the evidence I saw whilst I was there which for me is enough but obviously not for people on a public forum, no worries I have a license.