You speak as if the design has not changed over time. To be precise, the history dates back to American studies in the 50s and 60s which were not taken forward but that were picked up by Alan Bond in the UK and evolved, then later developed into the HOTOL design, and still later into Skylon/SABRE - with significant work undertaken throughout this history.

I think that there are some factors to give one faith that this won't just become another sad episode of unrealised promise to add to the history: the engine design has had significant verification along the way by authorities of the very highest credentials, is furthermore thought to be of sufficient promise to have attracted significant external funding, and finally has resulted in working hardware on the test stand that has not yet indicated that the expectations of success are ill-founded; it's getting to the point where the glaring flaw that some seem to want to be uncovered is increasingly unlikely to be found (I'd add personally I find it a little sad that amongst tech/space enthusiasts many seem so sceptical about SABRE when the facts suggests otherwise, and I wonder whether it would be a different story if it were an American project - I'm enthusiastic for anything that gets the human race off this rock, no matter where it comes from).

That said, I think it is important to separate SABRE and Skylon. The former as I've said is thought by many well credentialled people to have genuine promise, whereas the latter seems far more of a PR exercise to illustrate potential application of the technology. Nobody's actually building parts of Skylon yet as far as I'm aware, wheres they are building bits of SABRE and getting them signed off as workable.