Surely the only reason you need to overtake someone is if they are driving lower than the limit, so this would suggest you're speeding as soon as you overtake unless they're trundling. However the highway code does suggest you overtake swiftly so as to complete the move as quickly and safely as possible, one could assume that creaping past at wagon-5mph isnt safe or sensible. I'd hope to be pulled, rather than camerad in that instant. In your case I'd expect 3 points.
I once overtook a really REALLY annoying 20-in a 30, 30 in a 40,40 in a 60 driver I'd been stuck behind for 10+ minutes on a dry fine day. It just so happened that the time I chose to overtake (a widening in the road) there was a crowd of 10+ police standing at the opposite side of the road. I opened the taps, flew past the car (in a mass of noise) and pulled in completely off the throttle having spotted the police. And JUST as a I passed them my over-run fueling ignited with the loudest backfire I've heard in a long time Liverpool police must have wondered who was shooting at them - they all turned round to look lol. Fortunately no tickets from that incident!