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