I think it depends on what flooring you choose roses, as my friend has underfloor heating with a ceramic floor, which extend s right through into the kitchen and it seemed very effective when I stood on it. However, we chose a wooden floor, although I kept saying I thought I remembered from physics lessons that wood is a bad conductor of heat. I just don't thimk our underfloor heating is the most effective, the flooring firm who installed it worked jn conjunction with the large wellknown company who installed the conservatory. However, DC can sit and play on a warm floor even in the depths of winter. The oil filled radiator is quite economical I think and we have sat in the conservatory with snow outside, at a toasty 23C, not putting the heat on in the rest of the house until later in the day.
Hope this helps, we did think the u/f heating would be enough, but not in ours in the depths of winter unfortunately, but then I don't think it would be enough in the rest of the house either