You are right the numbers historically were quite low because of the difficulty in obtaining an IR. but thanks to the huge amount of work done by PPLIR and AOPA in relaxing the requirements the IR is a lot easier to achieve now and the numbers are massively on the increase.
As the Head of Training of an ATO we are seeing a big increase in the number of PPLs coming through solely for IR training.
Hopefully over the next few years we will see the uptake of the IR in the percentages that the U.S. has.
I am not a PPL however I use my Cessna SEP for a lot of private IFR flying.