Amongst the several dozen questions that spring instantly to mind, what on earth is the Mayor planning to do with all the Clapham traffic when it arrives in Canada Water? There are only three ways off the peninsula as it is, the one-way system is eternally congested and both Lower Road and Jamaica Road are full to capacity at busy times. It's a while since I've driven through Clapham but unless serious improvements have been made, things are much the same at that end. The tunnel would probably be destined to become one of London's biggest traffic jams.
As Sir Marc Brunel (Isambard's father) discovered when he had built the Thames Tunnel, which opened in 1843 to link Rotherhithe and Wapping, you need quite a lot of room for the infrastructure at each end of a tunnel to handle traffic entering and exiting it, an infrastructure that needs to integrate with local roads. Brunel never pulled it off, and the Thames Tunnel remained a pedestrian route beneath the Thames until it was converted to a railway tunnel (now still in use for the London Overground). The Rotherhithe Tunnel, which opened in 1908, did have the infrastructure for road traffic, but it required the demolition of a huge number of Rotherhithe homes in order to achieve it. So where would this infrastructure go, and what is Boris intending to knock down in order to provide a space for it?
I'm not convinced that Southwark Council would jump on this particular bandwagon - they are more focused on putting up income-generating tall buildings on any available space. Unless, of course, they see it as an additional attraction for developers who could advertise the proposed tunnel as a benefit for potential purchasers. Who knows? Getting into the minds of planners is quite beyond me.
I'd say it was poorly thought through, but this just has the air of not actually having been thought through at all. The mind boggles, quite frankly.