Not sure that the heat from the underfloor heating would be able to penetrate your whelping box, especially if its slightly raised off the floor. Also the idea of a heating pad is so pups can get on it but also move off it if too warm.
I like heating pads.
I agree you need space for yourself, my last litter was a nightmare and I spent the first 6 weeks with them in total, before I felt comfortable enough to leave them and after such a long time it felt very strange in my own bed after that. Also had CCTV set up so I could see and hear them 24/7