DFC,
You raise an interesting point about notice.
Of course, 99% of the Class D that affects the average VFR pilot is zones, not TMA, and the norm there is to ask for crossing clearance at short notice. Actually, most zone controllers (I am talking mainly London TC here, so the Thames, Luton, Essex, Gatwick crowd) do not really seem to want to hear from you as much as 10 minutes before, they seem to mostly take a tactical decision as you get close in.
But the Scottish TMA is a different matter, being under Area Control (I think). I wonder if the suggestion is that if I want entry to the TMA I should file VFR, rather than just arriving on hand over?
My suspicion is that a VFR FPL that mentioned going via TLA to Cumbernauld would be ignored by ScATCC, because the level would be VFR and the assumption would be as that made by the 124.5 controller at the head of this thread that the flight would be beneath controlled airspace.
I guess that the FPL could specify F085, but that subverts the whole point about VFR which is that you cannot be sure, several hours before, what levels are going to be available VFR.
So 10W and nats type guys...would a VFR FPL make any difference?