UFH retrofit - heat loss calculation

Looking at retrofit UFH over a 1970s uninsulated concrete slab and running through the MCS heat loss spreadsheet. The suppliers all claim minimal downward heat loss but I don't understand the maths behind their claims. Anyone an expert?

Given an uninsulated slab, and a ground temp of 12 degrees, mean water temp of 35 and a room temp of 21, the heat loss through UFH in contact with the slab is 2.5x times that of radiators?  dT goes from 9 degrees to 23 -> Heat loss is Area x U x dT? Area and U value stay constant?