In theory they would be going the speed limit, which means overtaking them would require breaking the speed limit and therefore the law. Yes it will be annoying for those that want to drive faster than the speed limit.
In summary: no, you're not allowed to speed in the UK when overtaking.
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.