Sketchy large employers like G4S responded by setting up tens of thousands of "Mini umbrella companies" [1] with directors in the Philippines, each company employing only a handful of people - allowing G4S to benefit from the £4,000 discount tens of thousands of times.
Sadly, exempting small operations from regulation isn't a simple matter.
Complex corporate structures enable plausible deniability. The CEO of GFS probably didn't know what was happening, but also probably didn't want to know whilst enjoying the low fees charged from the recruiters.
It's literally managements job to be aware.
Imagine if a crossing guard waves cars through an intersection as children crossed and goes "Well, you know, I wasn't driving the car".