The problem is the electrical noise generated by the led drivers (or dash cam processor) being "louder" than the more distant DAB/FM signal. Essentially have a DAB/FM noise generator attached to your van
If you fit an antenna that has more gain you'll just get more of both and I doubt it will help. Relocating the antenna may help.
Ideally cure the source of the noise.
The reason this happens is that LEDs tend to be driven by PWM signals and dashcam processors have a clock signal. Both of these use a square waveform. Mathematically a square waveform is made up of a series of different frequency harmonics. The upshot is any unshielded wire with a high frequency square waveform signal on will be radiating a great number of harmonics, some of which may fall into the frequency bands of FM/DAB
The reason ferrite cores work is that the high frequency signals induce a current in them and the back EMF of that current opposes the original current. Essentially putting a ferrite core around a wire blocks or strongly reduces high frequency signals, which is why sometimes they are called a choke.
To be effective they need to be placed correctly. The issue is high frequency signals bleeding out of electronics on to the cables supplying it power, those cables then act as antenna to radiate the signal. To be effective you have to block the signal as close to the source so the length of wire it is on has the least chance of radiating it. So you should place them as close to the noise generator as possible, which is why you commonly see them moulded into camera and laptop leads just behind the power connector.
