As far as I understand, it's illegal to overtake on a pedestrian crossing upon the zig zag lines.
Does this apply to cyclists want to pass slowmoving traffic?
I can see good reasons why it might not be legal.
AFAIK there isn't an offence of overtaking on the zigzag lines of a zebra crossing for cyclists, although overtaking might result in an offence of failing to stop for a crossing. The zigzag markings are there to indicate the crossing apporach and cannot be parked on.
As far as I am aware, you can filter on the markings, as long as you do not failt to stop for the crossing itself.
However, on most crossings, it wouldn't be possible to overtake (as opposed to undertake) on zigzag markings, as they are usually only painted on the kerb-side side of the crossing. Exceptions arise where there are two (or more) lanes either direction (in which case overtaking isn't an issue) or where the zebra crossing is actually two separate crossings, divided by an island, where a zig zag marking may be painted next to the 'kerb' in the middle of the road.
*Amended for clarification