Basically it is an SOP which is merely part of a LARS 'agreement'.
From the UK IAIP:
After leaving the Airways System pilots will normally be provided with a radar service outside controlled airspace by Farnborough ATC
so it's part of the radar service 'deal'. Certainly Farnborough may not insist that the aircraft remains in the hold, but in Farnborough's defence it's probably more convenient for all parties if it wasn't just wandering around willy-nilly whilst waiting for a vacant runway. It's all about how they 'package' it really, and, more to the point, how the deconfliction service is 'delivered'.