it is not against the law to be without a TV licence. It is not against the law to own a TV that is kept solely for watching videos, and is provably disabled from receiving signals.
But it is against the law to receive TV signals without a licence.
to prosecute for the latter it would be necessary to have evidence. evidence from a tv detector can only be used to "establish that an offence is likely to be taking place".
so the need to see a tv for prosecution purposes may well be true.