Reaction Engines in the UK spent over 35 years working mostly on that concept
(though when they eventually went bust trying to scale up last year I think they were focused on reusable space launch business model which is ironically more realistic)
No, they were working on the latter (skylon) most of the time, though the new management that came in after their £60M investment quickly dropped SSTO in favour of more immediate RoI applications. The passenger plane was LAPCAT which was a paper study commissioned by the EU. They did some interesting real work too, such as designing and testing a hypersonic engine combustion chamber that could reduce NOx emissions, which would be a big problem in any ‘conventional’ (eg scramjet) hypersonic engine.
