I didn’t realise people swapped duvets year round in the UK!? I tend to just get more scantily clad and keep the same duvet. In the winter I will sleep with a wooly jumper on
staying at my parents I had two hot water bottles as well as a jumper, PJs, thick socks and I’d still wake up shivering 
Anyway, same duvet, less clothes, feet out and eventually the window will be opened, we’re not quite there yet though. We tend to get a lot of screaming drunks on our street so it’s not always ideal.