It's a very interesting concept that does, indeed, build off HOTOL. Conceptually, the same sort of thing (single stage, close enough to orbit to need only a small kick stage) has been looked at before, and it is more attainable than true SSTO.
One problem is that the space business is now built around expendables, with a large and (until SpaceX) government-subsidized launch industry and big, long-lived, expensive spacecraft. The volume of large launches is therefore small, and consequently a big upfront investment in a launcher with low recurring costs is unattractive.
Another problem is that SpaceX has moved the cost bar, and is working on reusable technology, if not as flashy as Skylon's.
On the other hand, Skylon's technology would also be a way to get to hypersonic point-to-point flight without getting into the hairy aero-thermo-physics of scramjets.