So many points it's like a porcupine at PMQs.
> The spaceship’s carbon footprint for each of its passengers and
> crew will be about a quarter of that for a return trip from London to New York,
As Spaceship One only flew ~40km (straight up, granted, but starting at it's 60,000 foot release height and ending around 100,000ft) that's not quite so impressive as it sounds.
It's hard to know how much /less/ polluting the SSMEs (the Shuttle's main engines) could be, what with burning hydrogen in oxygen; you can all guess what the waste product of that is.) Same goes for most other man-rated and otherwise launchers in the west. The SRBs burn aluminium perchlorate, which is no fun at all to breath in, but take a look around the low scrub and lagoons around Kennedy next time you're stuck watching NASA TV during an interminable T-40:00 hold...
The really nasty propellant is hydrazine, which is pretty damn toxic (that's why curious cowpokes across Texas were warned not to go poking around any smoking wreckage from the Columbia accident.) However the Shuttle, like the other spacecraft uses of hydrazine, reserves it for in-flight burns. For instance the course correction manoeuvres carried out by planetary probes are generally made with hydrazine thrusters.)