Butt Ugly Martians limited time TV advert (2001/2002?)

I remember an advert about the Butt Ugly Martians that might be what you're describing, given I'm also from the UK. Weirdly enough though, I never hear any mention of this advert, so it could be because nobody cares about this show which is very understandable, but maybe it's also a UK thing? I sadly don't remember much about the advert itself since I would've been around the age of 5 at the time, assuming it was aired around the show's premier.

The most I can recall about it is just a big martian head talking against a dark background, kind of like some Crash Bandicoot: Warped time twister message, which to my knowledge, wasn't showing any products in the advert, just talking until the end. Going from what you said, maybe it was the main villain threatening people to watch the show on another channel or something, or he would destroy the world if they didn't? I could be wrong, but the franchise had pretty ambitious marketing, so I could imagine that at least.

Being scared by it is something I do remember though, there's two instances that I'm pretty confident in. I'm sure it appeared during the adverts either after or in the middle of an episode of Pokemon in the morning before school (my gut is telling me that it was on Sky One), and another some time after school where my dad saw me distressed at the advert, and decided to pull a stunt, essentially shit talking and yelling at the martian to go away, and changed the channel as he did, onto some gameshow which I'm almost certain was My Kind Of Music.