I'm unfamiliar with the UK manufacturers, but on this side of the pond they mostly use the same fabrics, except maybe at the very low and very high ends, cost-wise. Primaloft does require downproof (or rather, Primaloft-proof) fabric, or it will "beard." Continuous-filament insulation may not, depending on the construction.

In any case, the synthetic-to-down conversion is impractical, as the other posters have said.