In reply to Fredt:
Have used a Vango Comfort 5 self-inflating mat and that is very warm and comfy - but a bit of a fight to deflate and pack. Newer ones than mine have much improved valves and should be much easier. It's probably about as big a self-inflating mat as you'd want to take - anything much thicker is going to take up half your boot
Their flagship one is the Shangri La, which has die cut foam to save pack size, but empty your wallet.
Mostly we use an Outwell Reel double, which is very comfy and packs down smaller and easier, but is essentially a glorified lilo, so may not fit your spec.