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.
Was only trying to point out that skimming along by the kerb isn’t the recommended way to pass/filter. Maybe willmelling said that too.
More often than not I ride where you are, as traffic flow means motors are passing me.