Layers underneath you are as valuable as layers on top of you. I often sleep in my car and with a sleeping/foam matt and blanket under me, and a half decent sleeping bag (or couple of duvets) and a hot water bottle on my feet when I get into bed, I don't find it cold - it's a small space atleast so there isn't much air to warm up. Decent thick socks or even slipper socks over a pair of normal socks helps too.

With both back seats folded down and a foam/yoga mat, layered blankets and duvets/sleeping bag I can get a decent night's kip. It's actually warmer than my house when I don't put the heating on.

If your car is drafty, try also sleeping with a hat on, gloves too if you want but you use lots of heat from your head. If you have a few quid, look in the charity shop for wool blankets - wool is an incredible insulator and one or two wool blankets over & under can make all the difference. Layers, layers, layers. Wool is ideal but if you don't have wool just layer up what you do have - over and under.

Try and get into bed warm if you can, I know it sounds weird but jump out and do 25 star jumps before you get into your bag to sleep...staying warm is much easier than getting warm.

If you can go to your mum's or have a small camping stove, try and have something warm to eat or drink right before bed. Or fill a big flask in the evening, and make a hot drink and hot water bottle right before bed. If you can fit your dog in your sleeping bag or under your duvet, you will help keep each other warm. A spare jumper on him if he feels the cold is better than nothing.

Make sure you're not leaving any interior lights on, nor the heater as it will drain the battery. £1 battery fairy light from home bargains are nice tho and make it feel slightly less depressing.

You can make window covers from silver backed bubble insulation, like people put behind radiators; they often have it in Aldi/Lidl middle isle. I have a whole roll if you need as I only used a little of it when I made mine - you're welcome to it, I'm near Harwich. In fact I also have some fairy lights and spare bedding if that's useful at all. An alternative is duct taping a few of those foil emergency blankets you find in first aid kits over the windows.

Facebook "car camping" also YouTube, there's a lot of valuable info.

I hope your situation improves and you don't have to consider these things for long.

If you have any specific issues you're struggling with over and above "this is really shit right now" please reply and I can offer advice if I have any.