A few personal observations on this intersting thread.
Firstly, I agree that it would be helpful if the Highway Code specifically stated how to behave on a filter lane. The closest it seems to come is in the section on road markings, where the type of markings used at the start of a long filter lane ('hazard warning line': long lines, short gaps) refers the reader to Rule 127, which states: "A broken white line. This marks the centre of the road. When this line lengthens and the gaps shorten, it means that there is a hazard ahead. Do not cross it unless you can see the road is clear and wish to overtake or turn off." Clearly in the context of a filter lane this line is not marking the centre of the road, but its use does suggest that from that point forward, the road will become two separate carriageways with an attendant hazard. Also, the fact that it refers to seeing that "the road is clear", as opposed to "the road ahead", allows for it to be applied to lanes where the traffic is travelling in the same direction.
This is supported by the broader definition in Schedule 6 (Road Markings) of the Traffic Signs Regulations and General Directions (2002), which doesn't refer to these lines as centre lines, but states that traffic should not cross or straddle such a line unless it is safe to do so.
http://www.opsi.gov.uk/si/si2002/023113aq.gif (diagram 1004.1). As the filter lane nears the juntion it's serving, these lines become shorter and more closely spaced to denote the edge of the carriageway, so this is the point at which the filter lane can be regarded as being a separate carriageway from the motorway.
http://www.opsi.gov.uk/si/si2002/023113at.gif (diagrams 1009/1010).
On a separate point, any talk of "undertaking" being illegal or resulting in a mandatory ban is wide of the mark. As has already been pointed out, there are occasions where the Highway Code supports it, while also stating the correct procedure of moving into lane 2 or 3 and returing to lane 1 as soon as it's safe to do so. Crucially, there is no specific offence under the Road Traffic Act (1988/1991) of overtaking on the wrong side (to put it generically), so it is not illegal
per se. The defined offences in this situation are 'Dangerous driving' (which carries a mandatory ban) and 'Careless, and inconsiderate, driving' (discretionary ban).
'Dangerous driving' is defined as meaning that your driving falls far below what would be expected of a competent and careful driver, and that it would be obvious to a competent and careful driver that driving in that way would be dangerous.
'Careless, and inconsiderate, driving' means driving without due care and attention, or without reasonable consideration for other persons using the road.
Now, there is clearly (and deliberately) a lot of room for interpretation in those definitions, and there is always the further variable of whether a traffic officer views your manoeuvre as even being worthy of their attention. You will get some patrols who will pull you over just to make a point, but I believe most are level-headed enough to do so only if they are genuinely concerned about the standard of your driving.
I find it highly unlikely that someone would be charged with 'Dangerous driving' solely for passing another driver on the inside, even if they were on the same carriageway. It would come down to the manner in which the manoeuvre was undertaken, particularly if it involved aggressive or erratic driving, sudden lane changes, speeding excessively, cutting up other drivers, etc. I'm sure we've all seen this type of behaviour on the road and wished there had been police in the vicinity to take action.
Without the aggravating factors, the charge would be 'Careless, and inconsiderate, driving', on the grounds that you did not demonstrate "reasonable consideration for other persons using the road."
Again, I believe this would come down to how you had performed the manoeuvre. For instance, I often find myself travelling in lane 1 at around 70mph, with lane 2 all-but empty and a bunch of traffic in lane 3 all intent on getting someone up ahead to move out of their way by sitting behind them for long enough (and no doubt flashing furiously). In this situation, I would not hesitate to pass the lane 3 traffic in lane 1, provided lane 2 remained clear and none of the cars in lane 3 was indicating an intention to move into lane 2. In that sense, I would regard lane 2 as providing a kind of buffer zone, and I would not envisage being stopped for doing this.
It becomes more tricky if you try to pass on the inside in an adjacent lane. I will admit to having done this on occasions but have always taken precautions, with a view to avoiding a charge of careless/inconsiderate driving. Firstly, I would maintain a safe distance behind the other car for long enough for the driver to realise I was there, in the hope that they would move over to the left. If they stay put, and the lane to the left is still clear, I would indicate and move into that lane, but still stay far enough behind them so as not to be in their blind spot, as I find that moving over to the left can often prompt a previously inattentive driver ahead to follow suit. I would then hold position for around 20-30 seconds to give the other driver sufficient opportunity to realise I was there, and if they were still showing no signs of moving over – and we were not approaching any kind of junction – I would then decisively move past them on the inside. This is obviously still a risky strategy, and if stopped for doing it I would have to make a case for why I felt it was justified, but I would also feel that I had mitigated the risks to the extent that my driving could not reasonably be regarded as careless or inconsiderate. Of course, that's not to say that a traffic officer would agree with me, and as I type this I can almost hear him asking me what makes me think the driver ahead wouldn't just blithely pull into my lane while I was passing him, if he'd been too oblivious/ignorant to move over in the first place.
That said, I would never switch lanes and undertake as part of a single manoeuvre, as this is highly risky: it is likely to take the other driver by surprise and possibly panic them into braking or changing lanes themselves. Also, I would never undertake in an adjacent lane if the other car was travelling at or very close to the speed limit, as to do so would negate any other mitigation you might have.
So, getting back to the original question, if I were in a filter lane adjacent to lane 1 of the main carriageway, with a slower-moving car just ahead in lane 1, I would recognise the other car as a potential hazard, and only pass it if I was happy that it was showing no sign of being about to move into my lane. If the car was indicating left (or even looked to be veering that way), I would drop back and give it priority.
I would also expect the other driver to exercise similar care and not move into my lane ahead of me without first checking that it was clear and then indicating, though I accept that that may just be wishful thinking on my part...
