That's actually one of the biggest questions I have: if you have a mix of UFH and rads, you'll need two temperatures. This can be done, obviously, but my understanding is that (gas) condensing boilers operate most efficiently at the (lower) temperature used by UFH. Is this correct? If it is, surely it's best to have UFH throughout? Sure bedrooms only need heating at very specific times, but can't zones with a separate timer and themostats take account of that?
This whole area seems a real can of worms, in terms of the amount of conflicting advice out there....