Not sure what an extra £100 or so would buy you if you were to get one of the big brands. I’m sure they’re better but not £100 better. It’s not very breathable so if you’re exerting yourself it gets a bit sweaty
It’ll get you – generally – a better warmth to weight ratio, probably a branded fill like Primaloft (possibly the recycled version, Eco) lighter fabrics, a better cut and possibly a better designed hood etc. If you’re a mountaineer or someone who needs those benefits they make sense, if you’re someone who basically just wants a big, warm jacket for wearing after, say, a mountain bike ride, not so much.
Patagonia has a synthetic fill that’s as warm or warner for its weight than high quality down these days, but it’s ruinously expensive.
I still use a first generation Rab Xenon for winter emergency mountain biking use. It has the first super lightweight, shiny Pertex super-lightweight Quantum fabric, but is surprisingly warm and packs tiny.
Lightweight synthetic insulation, counter-intuitively, turns out to often be warmer than down equivalents, because the sheet-type construction eliminates all the stitched seams used for micro-baffled down jackets. Go figure.
If you do a search for Primaloft on Sport Pursuit, a whole bunch of stuff comes up.