Er... as per Wiki page
https://en.wikipedia.org/wiki/High-speed_rail_in_the_United_Kingdom under the heading
Heathrow Airport to Gatwick Airport - High Speed 4Air.
"In 2011, the Department for Transport confirmed that ministers were studying a proposal for a 180 mph line between
London Heathrow and
Gatwick Airports. The journey would take 15 minutes and provide the first direct rail link between the two airports. The plan could virtually make Heathrow and Gatwick into a
single hub.
[51] In 2018 a proposal was put forward by a British engineering consultancy,
Expedition Engineering, for
HS4Air, a high-speed railway line that would run to the south of Greater London, creating a 140 km link between HS2 and HS1 via Heathrow and Gatwick Airports."
There was talk of the above, even before the 2011 study and you have the following from UK Parliament that dates from the 1990:
https://hansard.parliament.uk/Commo...3-5769-4633-92a0-da0f665b1a85/PublicTransport
Roger King the MP for Birmingham Northfield states the following:
"I welcome the debate on railways and public transport policy. It is interesting that whenever railways are debated in the House, there is an inevitable comparison between the investment that British Rail puts into the industry and the investment that is put in by our continental cousins, especially the French. The comparison between British Rail investment and that of SNCF with the TGV concept is always made. It is true that the TGV is a glamorous form of rail transport and that every country would like one. With France's land mass and ability to drive railways through the countryside more easily than we can, it has a transport system that is the envy of most of us.
How modernised are the rural railways in the Pyrenees or in other parts of France when one goes off the main glamour rail routes into areas that do not have the benefit of such investment? There is no doubt that railway investment in the United Kingdom stretches from Penzance to Thurso. Wherever the railway is in operation, especially in rural areas such as mid-Wales and the north of Scotland, new forms of signalling equipment, radio telephone equipment and rolling stock have been introduced. That has transformed the railway system in the area. That is the sum total of our investment, which has reached all parts of the country. The French railway system does not reach every part of France. We must be sure in our minds that we seek from a modern railway system not necessarily high speeds of 180mph or 200mph, nice though that might be, but punctuality, cleanliness and reliability.
We still have some way to go, but I am convinced that the people on the east coast route, who are in the process of receiving a brand new railway system, as we did on the west coast route 20 to 25 years ago, will come to appreciate the terms and concept of investment that the Government, through British Rail, are bringing to a large part of the country. I warmly welcome that."
You also have from 2005, the following article
http://www.apt-p.com/BBCTTONTT.htm where the person interviewed engineers from the development of APT where the site owner states "I had many conversations with APT engineers, obviously glowing with pride that their child had finally come good. "Of course she could do 180mph if she were allowed to..."
While a flight of fancy that freight trains would do 180mph, it was certainly thought about for passengers trains.