Maybe they wanted to leave the UK.

Why come to Europe in the first place? Because it’s comparatively well-to-do.

Why the UK? Probably because it speaks English. I bet that you’d find that people coming out of francophone Africa tend to head for France.

I’d also note that at least during the migrant crisis, Germany was getting more than the UK.

They wouldn’t do it if there was a processing center in Calais, something that France suggested but the UK rejected.

With an approved asylum application in their pocket they could travel to the UK by safe means.