I always filter on my bike when i reach slow moving traffic, there is no such thing as a traffic jam when you are on a bike, thats the advantage of 2 wheels over 4. There is nothing illegal about the actual act of filtering, there is even guidance on how to do it safely in the highway code.
I'm sure that it looks reckless and irresponsible to most car drivers who have no concept of 2 wheeled transport. The levels of concentration and focus during such manouvres is vastly greater than that of someone sitting in their steel cage with the heater on and radio 2 gently soothing them on the journey home, these are the people who make unannounced and unexpected manouvres jumping from lane to lane to gain an extra cars length on the guy in front of them.
Might i also add that when your crawling along at 5mph in your car, when the guy on the bike is traveling at 20mph it may make him look like he is driving way too fast and in a reckless manner when in fact its not the speed thats the issue , its the lack of concentration and consideration for others that causes a lot of accidents, hence the ad' campaign, think bikes!!. it doesn't just apply to when your about to pull out of a junction.
All said though i have witnessed some crazy antics by other bikers and i'm sure that that does nothing for the defence here, but no-one really goes out with the sole intention of committing suicide when filtering.