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.
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.