A fleet of trucks just drove themselves across Europe

It is forbidden in Sweden.

Erm, no, no it isn't... I can't find anything official saying that it isn't, but I can find lots of discussion forum posts of people saying that it isn't legal. The only references to it being at all permitted which I can find are (a) that a long time ago it was taught that the fast lane on a motorway allowed speeding (which it doesn't), or (b) that a speeding fine can only be issued if your average speed is measured over 2/3 mile (therefore more than an overtaking distance) however the prevalence of speed cameras in the UK would seem to disprove this.

In summary: no, you're not allowed to speed in the UK when overtaking.

Is it?

If a driver needs to break the speed limit to overtake it suggests they didn't plan the overtake properly.

https://www.gov.uk/guidance/the-highway-code/using-the-road-...

Rule 163 "Overtake only when it is safe and legal to do so" is pretty clear that breaking the speed limit in order to overtake is not allowed.

On single lane roads though, as OP was referring to? I honesty don't know though because I don't drive much anymore.