To be fair, my previous statement was based on trying to make sense of the HC, and on the premise that undertaking per se is not allowed. By contrast it's clear that (again, subject to general considerations of safety), two-wheelers are allowed to filter, on either side. The problem is that terms don't seem to be defined anywhere.
I'm prepared to be corrected, but I think that it is very unlikely that an exemption to the general rules on undertaking exists for two-wheelers - i.e. that we are allowed in general to undertake on the left, say pedal cyclists downhill when a motor vehicle was travelling at a (low) speed limit. And I'm pretty certain that motorbikes, which are allowed to filter between traffic lanes, are not allowed to undertake at the speed limit. All vehicles are allowed to filter, for example when two lanes of traffic are moving at different (low) speeds, so I've always seen the acceptance in the HC of filtering by two-wheelers as more of a recognition that we don't always need a painted lane to have space to do that safely. Specific exemptions to rules for particular classes of vehicle are, I think, quite rare.
So some people talk about undertaking by filtering as being permissible, where I've tried to use undertaking only in its obvious sense of overtaking on the left, in which case it's only ever allowed where lanes are dividing into separate roads and so on. As I noted, the HC never talks about bikes undertaking, except for the case I cited where bikes are undertaking other bikes, the point being as I see it that both are probably moving freely, whereas bikes don't often pass freely-moving cars on either side. (I did once catch a veteran car rally while riding a time trial, so I overtook a few that day, but in the usual way on the right!)
It would, I think, be useful if the HC could define its own terms!