I’ve also done a fair bit of car kipping in Scotland and in a camper at Val Thoren without a working heater. With a reasonable quality synthetic sleeping bag you’ll be okay, either double up with lots of space inside or get 5 season bag.
Don’t park up too far off the beaten track, check what the local law allows and be aware of the weather conditions. A few beers inside and a fresh dump outside could leave you very short on oxygen.
As above, you will not be able to dry gear and warm boots, you can keep the inners in the bottom of your bag. Bear in mind that you will be in the car for a fairly long time so weigh up the cost of food, drink, warmth and company (delete as appropriate) at an apres bar compared to what you can sort out at a hostel taking some drink and food with you.