I can see you are in an embarrassing situation if you are a law-abiding person and don't want to be seen condoning the law being broken. I dare say you will find an entry in Yellow Pages or the Telephone Directory that will give a number you can ring.
The people who have responded saying that people do not like those who report on ordinary people to the authorities are quite right. If you are decent person, I would have thought you would want to look for somewhere else to live.
I believe in upholding the law, and therefore that people should pay their TV licences. However, I, along with many others, heartily dislike the TV Licence. It arises from a time when TVs were easily identifiable, whereas now they overlap hugely with other household items like computers that are not similarly taxed. Also, the TV licence, which has been steadily increased to meet the demands of a bloated BBC, has become in effect a "poll tax" which bears heavily on the worse off.
How do you know they haven't a TV licence? Also, if you are bothered that the house is not covered you can yourself buy one, which you should do if you want to have your own TV in the house.