Cycle campaigning

It's basically what happened in Holland, Belgium, Copenhagen etc. Some streets were simply closed to motor vehicles. Some were made "shared" - I think this latter is the norm, but I personally believe you need Strict Liability laws in place to make shared space work for cyclists and pedestrians.

I've discussed this idea with Oliver before. Hackney have actually done it with a few short stretches, such as the back road from London Fields to the Town Hall. The City have done it with that bit from Southwark Bridge to Poultry. Tower Hamlets have done it in Poplar. Counter-intuitively, models show that it can actually improve motor traffic flow too - there was a thought-provoking study that suggested it would be better for motorists if Blackfriars bridge was closed to them. I think the idea is that although the journey may be longer, it's quicker because it is not interrupted so many times by cars joining from newly closed roads, and in addition everyone's going the same way.
And of course councils have no problem doing this to make bus termini, pedestrian areas etc. No-one ever seems to protest it, which is the interesting thing.

Basically the way I'd do it is this - If a road is going to be earmarked for pedestrianisation/bus use/whatever rather than general traffic I would also allow bikes on it. Allowing bikes should be the default position and a traffic order explicitly prohibiting them should be obtained if a developer or council wish to exclude them. Obtaining one should require solid evidence that cycles would be a real problem.

If a road is going to be ruined by narrowing, making it one-way, closing off one end, closing an exit, etc, (such as outside the Roebuck) I would consider closing it to motor vehicles entirely. At first this needs nothing more than a road sign and maybe a plastic barrier such as those on the "ring of steel", then later it can be made pretty with planting, blockwork, etc. You could at that point install droppable bollards to allow emergency vehicles through, and equip the vehicles with the means to drop them automatically when running on blues and twos.

The next, and more adventurous stage would be to consider new roads for closure. For example, why not ban motor vehicles from the Strand? You can route everything down the Embankment via Northumberland Ave or Temple Place. Could we implement two-way running on Gower St and remove motor traffic from Tottenham Court Rd?
Can we close off Oxford St and route vehicles along Seymour/Wigmore/Mortimer/Goodge St?
We have seen this in things like pedestrianising the North of Trafalgar Square.

Just to make clear why I think the cameras idea is a non-starter:

The thing about average speed cameras is that they are splendidly and almost completely effective in reducing speed to the posted limit. But with such low levels of non-compliance they are not revenue raisers, and they are very expensive to install and operate, so it has to be economically worth it. They are designed to limit continuous average speed, not maximum speed in a stop-start situation.
On 15 mile stretches of the M1 it is worth it.
The average can only be measured on a stretch of road with no side turnings, no exits into shops, no on street parking, no driveways, etc in between each pair of cameras, as all these hazards will most likely slow even the most antisocial driver down to less than the 20mph average. Now it also has to be possible to accelerate from a standstill at camera A and reach camera B at such a speed that the average is over the speed they are supposed to catch you at, or there was demonstrably no need to put them on that stretch of road at all. If it is impossible to break a law you don't need to enforce it.
Again, on the M1 these conditions are met admirably.
Now even if there were stretches of London streets where, if the road was clear, it would be possible for a pair of average speed cameras to reliably catch speeders, most London streets are chock full, and average speeds are never anywhere near 20mph. As a result I cannot think of many (or any) London roads which could be effectively policed by pairs of average speed cameras. Too much distance between camera pairs and the average speeds will be too low to make it worthwhile putting cameras in, too close and you might as well have Gatsos.
The only cameras which could work are Gatso/Truvelo type fixed speed cameras, which I believe can only be sited somewhere there have been 3 road deaths in a prescribed time period. However, we all know that people speed right back up again as soon as they are past them, the same as they do for speed bumps.
The police have always said they will not enforce 20mph limits as they do not have the resources, which is why all existing 20mph limits are designed to be self enforcing by means of introducing hazards, with of course the express purpose of making the road more hazardous. But slower.