The answer to your question 'Are you ready to spend a few hours, or more, in the car on a bitter cold day?' is 'yes'.
I put a double duvet in the boot last week. Costs less than £20 and all the things you've listed together won't make as big a difference.
qjs
What's the duvet made of? Down-filled?
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I very much doubt it for £20.
Almost certainly polyester or other false material.
For use indoors on a bed I would consider down to be preferable, but for emergency use in a car polyester is probably better.
A down filled duvet is not only expensive but vulnerable to damp or becoming torn.
Polyester duvets are sold everywhere at low prices and are a good prep for a car, but too to bulky to carry on foot.