Using a two duvet system provides the flexibility to adjust your temperature in bed and thereby ensure a good night's sleep.
Two duvet system has summer weight tog (4.5tog), a spring & autumn weight tog (9.5tog) which can be combined for a winter weight of 15tog.

Slumberdown All Seasons 15 Tog (4.5 Tog + 10.5 Tog) Double Bed 3 in 1 Combination All Seasons Double Bed Duvet
5.0
Amazon.co.uk

Lancashire Bedding Made For You Two Couples Duvet Hot and Cold Duvet Set | Made in Britain 4.5 Tog Plus 10.5 Tog Duo (7 & 14 Tog, King (U.K. Standard))
4.0
Amazon.co.uk

Sarah Anderson Felicia Duvet Cover Set with Complementary Pique Duvet Cover, Bedclothing with Bed Sheet, 2 Pillowcases and Pique 100% Cotton, Made in Turkey
5.0
Amazon.co.uk