I think the news story is also that it was spotted on the M25 which is the orbital motorway round London that is quite busy and that it is, in fact, perfectly legal.
Edit: Also it is the BBC which has a track record of regarding unusual vehicles as entertaining:
Also it's mildy amusing that the police stopped it because OBVIOUSLY that's not road legal, but it turns out that actually there was nothing they could cite it for - unlike a good number of cars on the road.
Egregious violations of traffic laws and revenue traps notwithstanding, basically all traffic enforcement is just cops stopping whatever makes thier spidey senses tingle. If you drive something that is super odd or highly out of place you will get stopped by every bored cop. This is not the first or the last time this guy will be stopped.
To be fair to the police in this situation: most velomobiles would be more like electric bikes and therefore not be legal on motor highways, wouldn't they?
It probably wouldn't be the first time someone eccentric created something cool in their garage and not go through the motions of getting it properly registered.
Edit: Also it is the BBC which has a track record of regarding unusual vehicles as entertaining:
https://www.topgear.com/car-reviews/top-gear-technology-cent...
From the 'review': "Against: The crash protection is a bit fleshy"