We had overlay UFH, in the sense of pipes laid in grooved panels on top of concrete, in our previous flat, although that was with a gas boiler on a top floor rather than having concrete touching the ground. In the new house we were looking at the same thing for our ASHP, but we were in the end able to dig up the screed.
The panels give less good thermal contact than laying the pipes in screed, so you need a slightly higher flow temperature, or more closely-spaced pipes, to get the same heat output - but the spacing is determined by the panel grooves. So the higher temperature decreases the efficiency of your heat pump a bit.
There are conflicting views (between, at least, us and our Heat Geek installer) about the role of insulation between solid floors and the UFH. I think you want to minimise heat loss into the ground; he thinks you want to use the ground as a huge thermal mass to stabilise the internal temperature. It probably depends on how wet the subsoil under the house is, and on your ground floor's perimeter:area ratio.
Anyway, if you are installing a wet heat pump just as a secondary heater for just this one part of your house, it will be insanely expensive. You would be much better off replacing the hall radiator with a modern high-surface-area one, or a fan coil unit, just to put more of your boiler's output into the that space. And then if you change the whole house to a heat pump later the hall at least would be ready for it. Or insulate, or even just plug in an electric heater. You need to consume something like 25,000kWh of electric heat before a heat pump pays back, even with subsidy.
