Microfibre duvets try to mimic feather and down duvets in the way that they retain heat but they're far lighter and usually quite a bit cheaper too.
A tog rating of 13.5 is often suggested as the ideal rating for someone who requires a warm duvet but who doesn't sleep in a freezing cold bedroom. (If you've got ice on the inside of your windows, you probably need a 15 tog duvet but otherwise 13.5 is likely to be ideal).
Putting those two things together, and then seeking out a suitable duvet (with very good reviews) from a reputable retailer leads me M&S's 'Home Essentials' 13.5 tog king size microfibre duvet for just £22.50:
https://www.marksandspencer.com/microfibre-13-5-tog-duvet/p/hbp22502066