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Unless you live in a passivhaus, the space heating will be thirsty, and using direct electric resistance elements to do this will be wasteful as you could only ever approach 100% efficiency, or a ‘coefficient of performance’ of 1.
Using the same amount of leccy, whether from the mains or augmented by solar/battery etc to run a heat pump will achieve a much higher CoP, usually 3-4, meaning you get 3x the heat energy for a given input compared to direct resistance heating. So again, unless your house needs hardly any energy input to stay warm, you’d be mad to use anything other than a heat pump to heat your UFH.
ectoplasmosis-
Like I said before, it's entirely possible that we won't be able to get an ASHP since where we want to put the external unit might not be feasible . If that turns out to be the case we'll have to stick with a gas boiler for our heating. If that is the case then we may be able to afford PVs, which we could use to augment the heating system and/or heat our DHW, which would save cash and carbon. If that is the case, how would that be integrated into the heating system, since boiler-based heating loops don't have a tank that you can put an element in?
ffm
By electric, I don't mean ASHP, I mean water for UFH heated by electricity generated by PVs. I know you can get solar-powered UFH, I'm just struggling to visualise the system diagram for it. Where does the heating element go? In the buffer tank?