Primaloft is all well and good, but it is no where near as warm as a high fill power down jacket. But it is slightly better suited to UK conditions in that it doesn’t matter of it gets wet.
Montane Prism is a pretty thin jacket in the grand scheme of things.
From looking around, the only synthetic fill jackets that come close to down are the ME Citadel and the Paramo Torres.
Alpkit are one to look at when they bring out their new Filo jackets in December. Other than that, just get the one with the highest fill power that fits (don’t confuse EU fill standards with US). Get one that fits snug as it will keep you warmer, and realistically you wont be layering up much underneath a thick one!