[rjmunro]

> watching BBC and not something else

You're not allowed to watch "something else" either - it's a national tax on televisions, not a BBC subscription charge.

Historically the detector vans tuned in to the local oscillator frequency generated by the television's "Heterodyne" tuning circuits. In order to tune to different channels, the TV generated different frequencies and used them to shift the broadcast an intermediate frequency to where they would decode it. If you know the intermediate frequency used by the TV (I believe they all use the same one), and can measure the local oscillator frequency, you know the channel being tuned to. You may well still know the frequency being tuned to with a digital TV, but each frequency now carries a multiplex of several TV or radio channels.

https://en.wikipedia.org/wiki/Intermediate_frequency

[4rt]

Your RF knowledge is no-doubt sound, the people saying TV detector vans are impossible have no idea what they're talking about.

But, ultimately the tax isn't on TVs. The tax is applied to people watching broadcast live television and it's used to fund the BBC, ITV, Channel4 and also used to pay OFCOM which regulates all other media.