18 months !!!! Not even 18 years matey. 80 is a better estimate. keep to the Fen Causeway
I'd guess getting above the stratosphere would count, but that's 50 Km up and you'd need a substantial booster rocket to lift an airliner that high, especially one that would have to have a lot of shielding and be a damn solid pressurised container.
And you don't glide back that easily either. The shuttle has proved resistant to reliable (or cheap)solutions for heat protection over large surfaces.
If you genuinely think Branson can get answers to those problems in 18 months, I have some magic ponies just here. keep to the Fen Causeway
That space vehicle will just be a modification of SSOne, which Rutan won the X-Prize with. Since SSOne has already been to the projected height and back twice, I think it's not unreasonable to say they can do it.
It's worth noting that the Shuttle's biggest problems heatwise come because you return from LEO at about Mach 25, whilst SSOne (going only to 100km or so) tops out at about Mach 3.5.
