Porcelain is a tough tile which will not crack or break easily. they are very long lasting. vinyl is out dated and just doesn't look good. Your other option should be ceramic tiles. So it should be between ceramic and porcelain. I'd still recommend porcelain but ceramic is cheaper, that's the biggest advantage of ceramic. I bought these porcelain tiles two weeks ago, they look lovely. beautiful hexagon porcelain tiles

Ask your self if the kitchen is actually a good kitchen….. if you need a utility room next to the kitchen for additional refrigeration or storage or additional work surface then it’s not a good enough kitchen for you. Ideally the utility should be just a laundry room in which case it’s best near the bedrooms upstairs or at least near the bottom of the stairs.

We are coming to the end of a house renovation and we had wet UFH installed in all our rooms. It is a bungalow however, it is very unusual property with rooms below the bungalow so it is like a house in some respects. We have a mixture of solid floors and suspended timber floors and couldnt have a screeded system because of the weight. We had an ESP overlay board system installed and have found it to be very effective. Ours is run off an ASHP so I cannot tell you how it would work off gas central heating. Our UFH is on 24/7 controlled by thermostats in each room/area. We did install internal wall insulation on all of the external walls and window reveals and voids beneath the floorboards where possible and my understanding is that this is key to efficient and cost effective UFH.

I'd also be inclined to swap them as Jonathan suggested. Check fire regulations, as perhaps you do need to rotate the stairs to allow an open kitchen. (I'm in the Netherlands so can't advise on these.)