The sole job of the heat pump is to keep the buffer tank hot (~75°c). Fresh water is heated from the water in the buffer tank via an external, brazed plate heat exchanger to provide potable, hot water. This means there is only a small amount of stagnant water, in the pipes, so there is practically no risk of legionella. Super insulated tanks loose very little heat are simple and readily available, so they make more sense than, say, a phase change material, thermal store.

UFH is typically installed in the slab, which works as a cheap thermal store, but it has some serious disadvantages. It is extremely unresponsive, taking a day or so to bring a house up to a comfortable temperature. It relies heavily on insulation, to be efficient at all. By using a low profile system on top of the slab, or wooden floor, this can mostly be avoided. You can even heat the area quickly by boosting the temperature, since it will cool down quickly too.

This is also nice because it can be used on upper floors, without any need for screed etc. If you already have a reasonably insulated floor, it can be installed on top, only adding a few cm and without the need for digging up your existing floor. A leaking pipe, would not be such a massive job to fix, either. This setup allows you to upgrade in more affordable phases, rather than doing everything at once. If you already have radiators, you could use them instead.

@goibhniu we considered fitting a similar system a couple of years ago. I went through most of the same ideas. In the end, I stuck with a traditional radiator wet heating system. Adding insulation, replacing all the windows and doors with very efficient triple glazed units, and working on air tightness means it takes very little heat input to warm the house.
I was dead set on a thermal store, but it takes a lot of input to heat it and in summer we wouldn’t need that much hot water!