I believe I have found a solution to the problem of vehicles squeezing past us so they don't have to use part of the next lane.  I've tested it now for several months, and my experiences are 100% positive from a safety perspective.  It's simple & cheap.  I've attached photos, so you all can see what it's about.

Construction

You will need the following:

• A long, straight collapsible rod of some sort -- I used an old manual car aerial.  Car wreckers still have them, but less and less so, since the electric ones are much more common nowadays.  Make sure it doesn't have a pipe that it goes down into -- you want it to be as short as possible when it's fully collapsed.
• A bright piece of material -- I bought a yellow reflective vest from the $2 shop.
• Lots of duct tape.
• A rubber band.

Attach the material to the end of the rod (I folded the vest in half and wrapped duct tape around it and the aerial several times) and attach the rod securely to your bike.  This part may be fairly difficult for some folks -- I have a parcel carrier on the back of my bike so it was simple, I just ran the rod under the carrier and duct taped it on both edges.  If you don't have a parcel carrier, maybe it's worth it for your safety to get one??? :-)

Put the rubber band on the rod and just leave it there.  It's for bunching up the material when you're biking with the rod collapsed, for example along a cycle path.  Then the material won't get caught in your back wheel.

Legality

Before I did this I fully investigated it with the MoT.  I found a guy who works on the transport laws and asked his opinion.  First thing he said was Good on ya :-)  After some research, his answer was as follows: so long as the material doesn't touch the ground and the rod when fully extended doesn't stick out further than 1m from your bike, there is no problem.  Mine sticks out 75cm so I'm well within that.  I found that I had to trim the vest a few weeks after I started using this contraption -- the aerial had sagged a little and the vest was dragging on the ground.  But since then there have been no problems.  The reflective strips on the vest make a noise when they drag on the ground, so it was very easy to tell when it needed trimming.

Effects

Motorists are significantly more courteous when I have my "flag" extended than they were before I did this, or than when it isn't extended (for example, when I'm going for a very short ride and can't be bothered extending it).  They see that they can't get past in places where they would normally try and squeeze past, so they simply wait.  I always put my hand up to my right shoulder to thank them for waiting, and if they've waited a while, I do it again as they start to pass.  The odd motorist acts as if it's a huge imposition (some, even, when they don't have to wait to get past -- go figure!!!) but it's very rare.  Trucks in particular are very courteous -- although I haven't yet seen what happens when an artic truck goes past.  I suspect the contraption doesn't change anything about what the trailer unit does!!!  But then anyone who cycles on the road knows already to be very cautious of trailer units...

Experiences

I found that I had to re-educate myself a little in the early stages of using this.  I was so used to thinking of myself on a bike as very narrow, and suddenly I was a lot wider.  I found that it helped to think of myself as a slow-moving Mini.

Another thing I found was that I had to move out to pass parked cars etc, much earlier.  You can't simply move out suddenly with one of these things poking out from your bike!  If I'm away with the fairies a little and I realise too late that I need to move out (including within the distance that, with the rod collapsed, I would start moving out), then I simply brake and wait until it's safe to move out.

Clearly going between lines of cars waiting at an intersection is more difficult with the rod extended.  But you can always collapse it, use the cross signal if you're at lights, etc.

This contraption has made me so much less scared when I'm on the road.  And it's not a false sense of security.  It actually works.  I heartily recommend it to anyone who is as sick as I was of motorists who are prepared to take the lives of cyclists into their hands, just in order to get where they're going a couple of seconds earlier.