I understand it like this. If the traffic is stationary, you're filtering. If its moving then you're undertaking. And we all know undertaking is for bellends :)
Exactly what I was thinking. Being generous, there's a grey area where cars are moving so slowly they're as good as stationary because you can be sure you'll be past them before they have a chance to change their line. So if you're concerned about swerving, you're undertaking, not filtering.
So would the suggestion be left-hand side when cars are stationary. Middle of the lane when traveling at the same speed as traffic. On the right/overtaking when moving faster than traffic?
I understand it like this. If the traffic is stationary, you're filtering. If its moving then you're undertaking. And we all know undertaking is for bellends :)