Reading this thread has made me wonder where on earth I put our summer duvet last autumn? Hoping to be able to change over to lightweight in the next few weeks!! That said, I bought two winter duvets last year in Dunelm for the kids. Quite an upgrade for them but they're house trained now. Only 20 quid each but so much nicer than the crappy smart price ones they used to have. Both (duvets, not kids) have stayed fluffy and bouncy since October. They are natural filling but obviously nothing exotic at that price. DH and I have a JL one at present which was dear but tbh nothing special. If I had to choose, I would pay more for a decent pillow, but that's another story.
I'm happy with my pop-together JL one. I think it's a down one. When I lived in Australia the wool filled ones were cheaper than down, and seemed warm in winter and cool in summer. Ours was quite heavy, but it felt nice and comforting IFYKWIM.
Now, pillows...that's another story! Searching for a pillow that suits was like looking for the holy grail.
IKEA's pillows were best for us in the end. They stopped making the cheap-but-brill one and the nearest equivalent was their most expensive but they're fab. I like how their pillow range caters for back/front/side sleepers.