Another bit of context that's important is that at the relevant times for this, you're looking at almost certainly 4 possible channels for most of the population of the UK.
There's a few ways of viewing some of this
One is simply that it's a lie and a deterrent. Not so interesting on the technical level. Most likely true but a dead end for fun tech ideas.
The more interesting is to think about how this could work - certainly some of the people in BBC R&D have. I'm sure someone will mention Van Eck phreaking, which is extremely cool but more than you need.
A much simpler approach, and one that actually seems easiest is
> it was thought that they operated by detecting electromagnetic radiation given off by a TV,
looking at the diffuse light through windows/curtains. You then simply need to have a live TV feed tuned to each channel and see if there's a direct match between the changing light and your own blurred monitor tuned to the same channel.
You can make this even easier by doing it at certain times, when Eastenders or Coronation Street is on, as a vast amount of the population would watch these.