I can't say I've ever installed one of these heating systems, but as I work on new builds only rarely, that's not surprising.
Underfloor heating was popular in the U.K. for a time in the late 1960s/early 1970s, but fell out of favor.
When used with solid concrete floors, it also suffers from the same sort of problem as storage heaters -- Difficult to predict temperature and to control. That's a big point in the English climate where day-to-day weather can be very unpredictable.
