This states clearly in the intro that insulation is NOT required for
all ground contact floors. For my basement at 2m depth 0.8
perimeter/area ratio, floor value from the table is 0.56 and walls are
0.58 (R=0.5) for no insulation. This is pretty close to what the
u-value calculator said.

The problem is not with having the floor insulated for general u-Value
reasons. As you have found out, floor insulation is rarely that critical.
The problem is burying the underfloor heating in it. Because the floor is
now hot, rather than the 21C to about 10C temperature difference, it is
coping with something like 30 or 40C to 10C, so the heat loss could be three
times as much. Coupled with the fact that it is electric, I would say that
underfloor heating is not appropriate. Have you considered a different
heating method?

Christian.