Our design will be to just top up the heat requirement and not heat it all from cold. I would start with improving the insulation
a 1250ltr hot tub would require almost 47kW to heat from cold to 38ºC so no point in using Solar thermal for that. A 3kW immersion would take about 16hrs.
LOL, you would not believe how many times people tell us it doesn't work up here. We are at Latitude 57 which is the same as one of the biggest solar thermal plants in Europe, situated in north Denmark.
We do a lot of research & development work on solar thermal and believe me it DOES work...
For an UFH system to work efficiently you really need a good amount of insulation under the floor. Heat transfer goes in all directions and you want it into the screed and not down into the concrete slab/ground.
I'd start by looking at how well insulated it is. That seems quite a lot of heat loss. Is there any way you can add extra insulation? Normally the panel sides just have a thin sheet of foil lined bubble wrap type material. That could always be increased with putting rigid insulation between the...