Lunatics. I'd say there are some exceptions to the rule on undertaking. Sometimes I think it's okay to undertake if going really slow and cycling on a cycle path.
It just depends what's in front of you. For example, I will occasionally undertake on this stretch of road (with great care), as sometimes you get tailbacks all the way to
this position.. The shared use cycle lane encourages people to undertake. I feel more vulnerable filtering down the right hand side than going very slowly on the left. Obviously you don't undertake anything once the traffic starts moving or as you near the end of the queue. The hazards are cars turning into the drive ways or people pulling over.
edit to add: it does create a dilemma for some people when they encounter a bus parked up or worse still a lorry. For safety sake I tend to stop undertaking once I see a bus or a lorry. It's just not worth the risk.