All your gains were in the rads and less pipework.
Rada are under windows to aid convection.
Yes they were. I didn’t claim anything else.
To quote CIBSE guidance
In application, they [radiators] should be set below windows to offset the major source of heat loss and minimise cold downdraughts.
If you wanted convection as the result, you’d put the rad opposite the window so rising warm air there would not be CANX out by the cold air falling from the window.
If you have efficient double glazing and decent curtains, the advantages of having a radiator under the window are far less than old single glazed units, so there can be a bit more flexibility in siting. They were often put under windows to keep wall space clear for furniture.