There is no doubt that that filtering is legal in the UK:
Extracts from Highway Code
Rules for motorcyclists
Rule 88
Manoeuvring. You should be aware of what is behind and to the sides before manoeuvring. Look behind you; use mirrors if they are fitted. When in traffic queues look out for pedestrians crossing between vehicles and vehicles emerging from junctions or changing lanes. Position yourself so that drivers in front can see you in their mirrors.
Additionally, when filtering in slow-moving traffic, take care and keep your speed low.
(
Rules for motorcyclists (83-88) : Directgov - Travel and transport)
General rules
Rule 160
Once moving you should
• Keep to the left, unless road signs or markings indicate otherwise. The exceptions are when you want to overtake, turn right or pass parked vehicles or pedestrians in the road
• Keep well to the left on right-hand bends. This will improve your view of the road and help avoid the risk of colliding with traffic approaching from the opposite direction
• Drive with both hands on the wheel where possible. This will help you to remain in full control of the vehicle at all times
•
Be aware of other road users, especially cycles and motorcycles who may be filtering through the traffic. These are more difficult to see than larger vehicles and their riders are particularly vulnerable. Give them plenty of room, especially if you are driving a long vehicle or towing a trailer
• Select a lower gear before you reach a long downhill slope. This will help to control your speed
• When towing, remember the extra length will affect overtaking and manoeuvring. The extra weight will also affect the braking and acceleration
(
General rules (159-161) : Directgov - Travel and transport)
Road users requiring extra care
Rule 211
It is often difficult to see motorcyclists and cyclists, especially when they are coming up from behind, coming out of junctions, at roundabouts, overtaking you or filtering through traffic. Always look out for them before you emerge from a junction; they could be approaching faster than you think. When turning right across a line of slow-moving or stationary traffic, look out for cyclists or motorcyclists on the inside of the traffic you are crossing. Be especially careful when turning, and when changing direction or lane. Be sure to check mirrors and blind spots carefully.
(
Road users requiring extra care (204-225) : Directgov - Travel and transport)
It is implicit in Rule 88, that filtering on a motorcycle is an acceptable way of “Making Progress”. Also, there is an onus on other road users to beware of such activity by motorcyclists. There are many debates all over the web about the relative merits of filtering, some are extremely anti & others are excessively pro.
The most sensible advice I found on the web was the 20 / 20 rule. The basic premise being; do not filter passed traffic which is moving at 20 MPH or above & do not exceed more than 20 MPH above the speed of the traffic that you are passing.
However, it must be down to the individual to determine if they wish to filter per se and whether it is safe to do so in any given set of circumstances.