SABRE
@awblain
If you take the time to read the discussions that I pointed to you'd have most of your doubts addressed better than someone like me can. I'd just be repeating them badly. It depends on whether you want to give it a chance or are set on imagining a SpaceX-only future.
@dead_pan
As for millions of welds - I'm not 100% sure what that's in reference to but they have built full size head exchanger modules using brazing to let them perform many welds at once. The heat exchangers are the super-duper-technology-that-makes-it-all-possible and are indeed expensive and difficult to make and that's what they've spent many years developing and what they aim to build themselves while the airframe etc is taken on by a company big enough to do that.
'Flogging it to the Chinese' appears to not be on their agenda but this doesn't mean they will not look outside Britain for things they can't get done inside. e.g. the latest news item is that RE has signed a Cooperative Research and Development Agreement ('CRADA') with the Air Force Research Laboratory.