Undertaking / filtering on left?

The Highway Code is advisory. The only bits that reference actual laws (RTA, TSRGD etc.) are prepended with "You MUST".

It's not exhaustive of all laws relating to being on the road, but it's a pretty good summary of all applicable laws.

So, if there is nothing stating that you "MUST" not filter on the left of vehicles, then you can be fairly certain there is no law against it.

I would say that it is pretty far from being vague on this.

Yes, undertaking cars is not unlawful. It is, however, as everyone knows, often not 'OK'. One thing that novice cyclists often have to learn is that you don't have to filter to the front at all costs (which cyclists do along the nearside more often than not). It is usually just as good to take your place in the queue, especially if the queue isn't long.