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Do not do this. It will be ruinously expensive to run. Seriously.
Electric UFH is fine in a small bathroom, turned on for a short while whilst having a shower just for underfoot comfort, but that's it. You really, really don't want to be using electric UFH to actually heat your living spaces, even with massive amounts of insulation, at today's leccy costs.
Look into air-to-air or air-to-water ASHP. Check out the HUG2 government grant scheme; if you live in a qualifying postcode, you'll almost certainly be eligible for a free of charge ASHP install without any means testing.
Electric Underfloor Heating throughout (no rads) in a flat that has no gas supply, opinions?