Filtering is not mentioned in the Road Traffic Act so therfore is not illegal, but not being mentioned does not make it Legal.
Police can and will prosecute if the manner of riding is considered in their professional opinion careless or dangerous.
Insurance companies usually will impose at least 50% of the blame on the rider in the event of an accident when the rider has been filtering.
Even if not filtering at the time it goes to the riders state of mind and riding behaviour, taking risks and not riding appropriate according to the Road Traffic Act. (Insurance companies consider filtering as a illegal at it's not mentioned)
I will happily filter when the traffic is slow, but with caution and I pull back in around junctions as its far too likely for someone to move out of lane 1 to leave room for traffic to pull on (more likely lane 2 is moving 0.02mph faster)
After traffic speeds increase to 30mph I will get into a lane. And I'll never filter more than 15mph over what the traffic is doing. Even then you little time to react.
I do filter with wide panniers, I've never hit anything with them, but sometimes cars will have to pull out of my way as I don't like to sit and wait for some woman in a Chelsea tractor to finish doing her makeup in a queue of traffic.
(1000cc V-Twin @ 5,000rpm with straight through cans gets a bit annoying inches from your head.
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Loud cans (mine are very loud) do sod all to attract attention of those ahead of you.
Your exhaust is pointed rearwards and so the majority of the sound waves travel away from the bike to the rear.
Most of the time a car driver will be made aware of your presence via exhaust sound with just enough time to look out the side window as you go past at 10mph (over their speed).
An awful lot of drivers are so wrapped up in what they are thinking, txting, changing radios to notice sirens and big blue flashy lights.
Do for a moment think they will in anyway be alerted to your presance with a slightly noisey motorbike.
Loud cans do not in fact save lives... they just piss off your neighbours.
I only like a few of my neghbours and they are bikers so f### the rest of them.