Let me just add that I think I misread/misinterpreted oapilot's original post. I thought that he was describing a scenario like my case 1, whereas in fact I think he was describing case 3. If so, my implication that London was using ambiguous instructions/phraseology in this case was unwarranted -- I apologise.
I would still stick by my assertion that, although it is clearly right to question any uncertainty about clearances as coracle suggests, in general it is important that instructions/clearances are completely unambiguous without the need for further dialogue.