I believe you can get an electric blanket that you plug into a power pack.
For me if it's cold I'll keep myself wrapped up, decent boots, decent socks, longjohns, with joggers over the top, thermal top with a t shirt, hoodie, coat, wooly hat and gloves.
In the bivvy I don't mess with heaters, I'll take a hot water bottle for the sleeping bag π tbh, if it's too cold I just won't bother going π€£