Evidence from TV Licencing Vans have never been used to attempt to convict anyone who allegedly failed to pay for a TV license.

You could infer from that, that the vans were for show and a shock tactic only.

It's not impossible, I'm sure.

Back in the CRT days it was reasonably possible to detect a TV program from the intermediate frequency from UHF stations, but more difficult (but not impossible, see TEMPEST) due to direct conversion by SDRs. These side channel attacks can be found here: https://en.wikipedia.org/wiki/Van_Eck_phreaking

Sensitivity of the equipment and picking up the signal from the roadside would have probably impossible in the 60s-80s. But again it's a capability you don't want your cold-war enemies (or allies) knowing about.

Also, being able to prove that the signal was the defendants, in a way that would convince a jury might have been difficult.

I'm sure some experienced radio hams/RF engineers will weigh in and correct me if I'm way off base here.