Sidesleeper here and been happy with my John Ryan Origins, had it since sometime first half of the year. We have an extra 3" cool blue foam topper also, really comfy.
Not tried a bed in a box to compare it to but the construction didn't look as good on the ones I saw.
not sure all foams are warm - Tempurs are warm too, but I don't know if Dunlopillo would be, they have airation holes.Foam vs springs is a personal preference. Most of whats been recommeneded so far is foam (simba etc) but while comfortable I find I sweat too much on them as they generally arnt breathable. If your looking brethable then your looking at springs and I recently got a Millbrook Enchantment 3000
I saw about 100 different brands of similar looking mattresses when I was looking to replace mine a week or two ago, prices are all over the place for seemingly the same thing!
In the end, I went for this one which won some Which award, and had some good reviews online - can't fault it and incredibly comfy. I would say it's more comfortable than the Premier Inn ones (I stayed in one again yesterday )
https://www.dormeo.co.uk/dormeo-memory-plus-mattress.html
Whoever made that website needs their head testing...