when does filtering become DWDC?

[fireblake]

Filtering itself is not illegal. Even undertaking (overtaking on the left) is perfectly legal now.

If you filter or undertake it is for the police to prove that your standard of riding fell below what would be considered acceptable.

It will be DWDC if you weave from lane to lane, suddenly cut across the front of vehicles, or ride too aggressively between them.

So filtering on its own cannot be DWDC. If you ride smoothly and safely, do not take risks, and do not compromise the safety of others then you shouldn't have a problem. Speed does not make any difference here.

If done sensible it is ok for example this chap  :lol

This chap reminds me of me when a courier in London. Only joking.
I think I've seen this bike for sale, low mileage, never been thrashed, slightly used limiter and horn