You lose a very large % of the body heat from your head, and having to sleep with cold feet makes you feel miserable.
They lied to us on that one
. It's based on skewed findings from biased tests. It's based on a person being efficiently insulated for their entire body, excepting their head. So, obviously, the head would lose a lot more than other areas. If you insulate the head but then remove insulation from elsewhere that then becomes the area of greatest percentage heat loss. That said a nice dry thin beanie can give you a nice little boost if the temperature is challenging the bag's low end rating
As Janne alludes to, even in dryish UK weather a day's activities will cause a level of foot dampness. Sleepping in damp socks can be a misery! I tend to sleep sockless, with the damp* socks being kept in my bag near my core to dry overnight. * DAMP, not wet. Wet socks are wrung out and hung to dry as I'm best able and then dealt with slightly differently.
As Janne says, a wool beanie can be a useful addition to your sleep kit if it's cold. Same goes if you don't like your head restricted by the sleeping bag and feel the need for a bit of coverage. Personallly, I tend to sleep with a hand under my head ~ so I also tend to carry a pair of loose mitts to my kit
I use a Klymit kip mat ~ basically a hi-tech inflatable crisp packet with absolutely zero insulating properties! If it's cold I put a ccf* mat under it ~ cheap and light *closed cell foam. As mentioned by others, if your pad inflates make sure you have and take, a puncture repair kit
If you do find that your new bags are a bit light on warmth I'd first look at insulating underneath you ~ your weight comresses the insulation and the ground 'sucks' heat away faster than air does (you mention a tent, so retention of radiated heat is less of a concern). One of the tricks I use is to put one or more blankets (fleece or wool ~ or a poncho liner) down first and the sleeping pad on it/them. The down side is that blankets are bulky and can be heavy too
Massive generality alert! ... Be aware that there tends to be a disparity in cold tolerance between males and females. This disparity is about 5 degrees C ~ so if your new bags have a comfort rating of 5 deg, count it as a comfort rating of 10 gegrees C (If I remember the sciencey bit correctly the feeling the cold more bit is down to women, generally, having a whole lot more nerve endings on their skin than men do)
If it's a bit nippy I tend to find that if I can keep my kidneys and shoulder/neck area warm then the rest of me doesn't grumble too much. After that a dry beanie and/or mitts helps maintain comfort
And remember that it's you who warms the sleeping bag, not the other way around ~ a bag may or may not not hold the heat well, but if you go into it physcally cold then you will likely remain cold!
