Admittedly I'm in a different country, but we get cold over winter. Usually my internal temp without heating is 15 degrees or so. Out where I grew up, overnight temps hit negatives. That house was old old old, uninsulated, lots of gaps, only one heated room. So whilst we might not be UK temps it sounds like internal temps are similar.
I don't use heating. Too costly. On very rare occasions I will turn the reverse cycle air con on to heat up for a little bit. I just find the air gets stuffy with heaters on.
Personally I prefer feather doonas as they are light and toasty. I change the weight of doona covers depending on the season. Winter heavy jacquards etc, summer light cotton. Given our warmer temps across the year, mine is 50/50 feather and down. Higher down content is warmer. For feathers to work, they need to be as close to your body as possible. If you want additional layers (I would recommend wool) they go on top of the feathers, not under. I visited home recently, a brisk max 10 during the day. I was warm in bed but the blanket combination was so heavy I was pretty much pinned in place all night. Definitely prefer the feathers!
You can also do flannelette sheets (I'm not a fan, I get too hot), flannelette pjs and socks. Tuck pj pants in to socks to stop drafts. A singlet / thermal top under the flannelette top could also help if cold overnight.
During the day, I dress for the temps. Nothing too heavy though. A pure wool jumper if I get cold, but normally two fairly light long sleeve tops and not overly thick pants. Warm socks - wool or bamboo, again tuck pants in. Inside temp is a pretty constant 15 during the day.
I always have a blanket on the couch - I'm under it as soon as internal temps are below 25. I've had two blankets over winter - they're synthetic and not very thick. Wool would be much warmer.
We're prone to drought over here so conditioned to short showers 2-5 mins max. It's tempting to stay longer in winter but hot showers dry your skin out, so it's best to be quick.
Pets - ever since my staffy rattled through the night on wooden floors (he was shivering), the pets have pjs. They get a choice on or off but pretty much come running as soon as they hear the rip of the velcro. They have lighter polar fleece for day time. Depending on temps, they may keep the day pjs on and get a second coat over the top at night, or just get a single warmer coat at night. Pjs are on once outside overnight temps are single digits.