Just caught up with this thread (been on my hols)
You've got to look at regional differences here too.
Filtering in London is seen very differently as for instance in Blackpool. I was taught to ride in London by a city riding veteran, and its brutal in comparison, but the car drivers are used to it. Ride like that in Blackpool/Preston, and you'll get pulled by the law, or not seen and probably knocked off!
As for legal side of filtering. As long as your not doing stupid stuff like making people take evasive action, or being dangerous/endangering other road users, then it's a case of just be careful.
The speed is up to you, just think of how it looks filtering at 30,40,50 to other road users who maybe are not bikers, and could very well be an unmarked cop car with video fitted. Condemning evidence to a layman in court.
Ride safe guys!