Flatmates don’t want to put the heating on, what should I do?

So for context, we live in a pretty badly insulated house and sometimes it’s colder inside than outside. My flatmates don’t want to put the heating on due to costs and I don’t know what to do. I’m freezing during the night and I’m tired of getting home and having to spend all evenings in bed, wrapped up. We talked about it before, but every time I’m being told ‘oh no, we’re gonna pay a lot’. I’m in a position where I could spend extra money for heating and was thinking of suggesting that I pay half the bill and the other two split the other half between them. I’m just concerned we’re all gonna get sick from the lack of heat.

Edit: Thank you all for your responses, some really good tips, I appreciate it. I will talk to them about the negative impact it will have on our health, the house itself and start putting the heating on every morning and evening when we're all home.