HOTOL is not just its powerplant. SABRE's integrated air liquefiers are just one of the new technologies needed to make it work at all, never mind carry a payload worth spitting at.
The SSO spaceplane concept has other flaws. It carries into orbit all the dead weight of a retractable undercarriage capable of supporting the fully-laden machine, along with the big wings needed to get the whole lot off the ground at a realistic takeoff speed. With minimal range in atmospheric cruise, the window of available orbits for minimal fuel burn would be restricted. Both HOTOL and the Reaction Engines SKYLON share these same flaws.
A carrier mothership would have reduced or eliminated these flaws, greatly increasing the maximum payload and/or allowing a smaller, cheaper HOTOL. It also increases the range of atmospheric flight, allowing more optimal positioning of the spacecraft to reach its designated orbit with minimum fuel burn. Bristol Spaceplanes propose a similar two-stage design, though they place the separation point farther into the flight trajectory than I did.
I didn't find reference to Bristol's preferred rocket fuel, but a mothership could also allow the inflight topping-up of an orbiter's liquid hydrogen tanks to replace the fuel that boils off during the early stages of flight. That alone allows an environmentally-friendly orbiter to be 10% lighter - or some of that 10% can be converted to extra payload.
Just think what a two-stage vehicle could do for the payload of a SABRE-powered orbiter. Or, think what SABRE engines could do for the operating economics of the Bristol Spaceplanes. I suspect that is the ultimate vision.