Here We Go...
So far as I can see, there's now no unproven technology in the HOTOL / SKYLON concept. Jet engines work, rockets work, the precooler now works, supersonic aerodynamics are understood, space flight control is understood, heat shields for reentry are a done deal. It's all there.
Ah, hang on; this being a British thing there's the significant question of whether a decent cup of tea can be brewed and served in a porcelain cup and saucer at all points in the flight profile of such a vehicle, complete with a digestive biscuit. (Yes, I'm British. These things are important). Pretty sure they couldn't on the Space Shuttle. What a waste of NASA talent.
So it might actually get built. Now that'd be quite a thing, and rather more showy-offy than anything anyone else is doing. Can't wait!