Just because the technology exists, doesn't mean the BBC actually used it. For one thing, it would have been far cheaper to put out the word that the vans are on the streets and patrolling, than to actually buy them, equip them and staff them with trained personel that could operate the devices.
Alternatively, perhaps a few of those vans really did exist and the rest was smoke and mirrors- rumours, fake vans without any active technology, etc.
Note that the reason I'm finding this likely is that the TV Licensing people are known for using psychological warfare tactics. For instance, I don't watch TV [1] and yet I periodically receive threatening letters from the TV licensing authority telling me that if I'm found to watch TV without a license I'll be fined, etc. Clearly, they send those letters to all addresses in the UK that don't have a license, under the assumption that most addresses without a license actually need one and that by sending out threatening letters en masse they will scare some of those shirkers into compliance- regardless of how many people who don't actually watch TV they end up threatening in the process. I've also seen some really startling public campaigns with posters showing bullseyes on houses, creepy slogans about being watched and so on.
Their tactics are a veritable nuisance and their mass mailing campaigns may or may not have the effect they want, but they sure have the effect of bothering random, uninvolved people with threatening government spam.
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[1] Er, well. I do occasionally watch TV shows, or rather clips thereof- on youtube. But that's not covered by the licensing. You need a license if you're watching TV programmes as they are being broadcast. So for example, I had a TV set for a few years that I used exclusively with a PS2 and I didn't need a license for it- but I'm pretty sure that if I had ever been visited by one of the TV licensing agents, I'd have been forced to pay anyway. How exactly do you prove that you have a TV set but don't watch TV? Most modern houses have aerial plugs - mine sure does. The only thing that really stopped me from connecting the TV set I had to the aerial was that I basically dislike TV. How do you prove that to an agent hell-bent on collecting?
