As was described to me by bike cop. Filtering in the UK isn't expressly legal, or illegal for that matter. It's a matter of judgement. If there is an incident then almost certainly you will be regarded as the offending party, and could be riding without due care or what ever other charge can be dreamed up.
If you are gradually moving between stopped or slow moving traffic doing barely more than the other traffic it's fine. Filtering at say 20..30 above the traffic is illegal. Filtering at speed on say motorways unless stopped slow traffic is illegal. But the police would be hard pressed to catch you and point out the error of your ways.and it would be tricky for the Police to register that with the courts.
Speed cameras may change that, but seeing as nearly all are looking at the front of the vehicle....
But it's clear that next may I will need to be very conscious of Dutch, French, Belgian and especially German road law. If only we knew someone legally qualified to provide the full SP and lead us not into temptation
