Quite possibly, but you would likely not get prosecuted. If you actually want to do online GSM decryption you need a lot fancier software radio than this thing, because GSM involves a lot of algorithmic frequency hopping/multiplexing.
Also there is no realistic practical way to passively detect a passive detector. Plus how would they distinguish you from someone on a normal mobile phone?
This is well known to be a a lie. The vans are fake. It's basically just extortion.
Just think of it as part of our eccentric british ways.
EDIT: just to add, I pay my license fee, happily, even though I dont have to because its not required to watch the iPlayer streaming service, which is how I watch almost all my TV. The fee is pretty cheap considering the sheer breadth of services the BBC provides. The UK would essentially fall apart without it culturally.
It may be fake, but it is possible. All televisions have a number of local oscillators for super-heterodyne reception. With sensitive equipment, the radiation from these oscillators can be detected because they are not well-shielded.
This is the case, but this technique will not allow you to distinguish between my television which I have paid for, and that of my nonpaying neighbour which is 12" away on the other side of an internal wall, on the 20th floor of a towerblock with hundreds of televisions in it, from the street. Which is what the Television Licensing Authority (the BBC dont actually have any involvement in collection or enforcement) claim they can do.
There was a television programme in Finland years ago, where they showed one of those "detectors" being used inside a building to find unlicensed televisions. What was visible of the device looked like an oven thermometer. It was the subject of much fun-poking, and there were screenshots floating around, but I can't find any of them now.
Also there is no realistic practical way to passively detect a passive detector. Plus how would they distinguish you from someone on a normal mobile phone?