For online radio, there are actual counts that change constantly.
For FM radio or equivalent, there are two methods: diary-based or PPM-based.
Here’s how the diary works (the UK model is “RAJAR”):
The PPM – a Personal People Meter – “listens” to what you listen to, spots the radio stations, and every night dials back to base and tells the research company what you’ve heard. Much like a diary, except it’s totally automatic.
Media UK lists the most popular radio stations in the UK: but the answer’s probably not as straightforward as you’d think…