The Reaction Engines Ltd (REL) web site has this to say:
"The resulting increased system performance enables aircraft-like horizontal take-off and landing operations, which reduce cost, infrastructure, and mission timelines while increasing responsiveness and system reusability. SABRE class engines are applicable in both multi-stage and single-stage architectures."
REL have been distancing themselves from SSTO as the first application for several years - you'll be hard pressed to find any mention of Skylon on their site now. A 30th Anniversary tweet showed a badge/patch including their current TSTO concept -- complete with (the other sort of) aerospike:
They have participated in a number of studies using SABRE engines as the first stage, including those with the Air Force Research Laboratories
and Paris University/CNES
Latest REL TSTO concept :