Go look in the AIP and read what it says about (not) entering CAS if you haven't yet received a specific joining clearance.
I am very much aware of
what is says;
Those flights, that have not received an ATC clearance, should not enter controlled or advisory airspace unless an overriding safety reason compels entry.
Note that the word "should" is used.
A G5 departs Farnborough for say Rome with the weather RVR400 OVC001 and has a commsa failure before they get the clearance to join.
Give me a safe option for the flight that does not involve controlled airspace and that you would expect the Australian crew to be familiar with.
Even bumbling round low level to somewhere like Manston or dodging everything all the way to Exeter would in most people's mind be more dangerous than doing what ATC should expect when they see the 7600. i.e. the standard international communications failure procedure and operating in accordance with the current flight plan.
At least with Farnborough, you will have apre-note of the departure. Imagine something departing from a VFR airfield further away and you don't have the details. First you know is when that 7600 which has suddenly appeared crosses the boundary line in the climb to their cruise level (because the 7 minutes is up).
You can take it that if I departed Farnborough (or Fairoaks or elsewhere) and had a comms failure in IMC before you could give me the clearance I would not be faffing about low level in crap weather trying to drag out the jeppesens and find an aerodrome with suitable facilities which is not in controlled airspace. I would be doing the standard IMC procedure and yes, I will enter controlled airspace. The simple reason is because I deem that to be the more safe of the two options.
Until there is suitable airspace and procedures, the risk of the unexpected radio fail joiner will continue.