Re: Something else to add to the Brexit Britain Epic Fail Compilation.
Specifically the lower air pressure at altitude rather than the closer distance to orbit (30,000ft out of 500km is pretty negligible)
Rocket motor nozzle shapes are very different for a rocket burning at sea level air pressure and in vacuum. Starting the rocket at 40,000ft, with 25% sea level air pressure, means you can design a rocket nozzle much closer to optomised for vacuum