You frequently receive correspondence from people referring to the problem of 'CLOG's (Centre Lane Owners Group). That will frequently lead to retorts about 'undertaking' and its illegality. I'm not sure this is right. I don't really like the word 'undertake' in this context anyhow; 'overtake on the inside' is what is illegal, if performed in the manner in which a normal overtake is performed. That is, a movement from a right-hand lane to one to the left with the intention of overtaking another vehicle. If you move past the other vehicle without adopting the actions that amount to an overtake manoeuvre, then surely you become a lane of traffic, albeit just one vehicle long, moving faster than the lane to right, and so is not an illegal overtake at all.