<<I have fed this back to the Twr before... If you call an aircraft to GA and acknowledge, then fine, but please do not expect on it to do anything other than the Std GA. You might be able to pass it subsequent Hdg/Ht instructions, but please don't assume these will be heard / actioned.>>
OK... I'm out of the loop but, Nigel, don't you realise that this might cause very serious problems? I had plenty of go-arounds at Heathrow which required immediate ATC intervention from the "standard" because of other traffic and nobody ever cried out to me that they were too busy... If the headsets are a problem they need fixing fast. Conflictions might still occur, no matter how the GA instructions are worded, and ATC issues instructions in the expectation that they will be acted upon..