Okay, a quick Google search reveals this document which dates from 2007, ie five years before the document that I linked to at post #336
Here are three quotes from this document as examples which show the importance of Heathrow, as do the diagrams in the document.
"A high-speed railway between London and Birmingham, with links to the West Coast Main Line further north to link the North West, north Wales and Scotland, provided with direct connections using a spur into Heathrow airport, is what is needed to maximise value for money for High Speed Two. The total new route length, including the connections to the existing network and High Speed One, is 150 miles."
"Greengauge 21 believes that it is very important that the route is developed in such a way that it can also serve Heathrow Airport. With Open Skies now agreed, Heathrow’s role as the leading international gateway in Britain will be reinforced: the country as a whole needs fast links to it. Heathrow’s value can be enhanced by having high-speed rail fulfil the role currently provided by wasteful, environmentally damaging short-haul flights, from the near-continent as well as from the northern half of Britain."
"From the route thereby created, provide in each direction for direct centre of Birmingham and with the capacity-enhanced (four tracked, Trent Valley) section of the West Coast Main Line with a fully segregated route, capable of generally supporting 300km/h high-speed operation interchange-free access to Heathrow Airport capable of supporting high speed services both to High Speed One and to the locations served by High Speed Two."
I can't find any previous research giving meaningful route considerations prior to this, but as in post #336, there was clearly a perceived need to do a route feasibility study five years later, as though it hadn't already been decided. All coincidence?