I am not quite sure what you are asking here. I don't understand "filled under pressure"? All sealed systems are pressure filled.
Where exactly the boiler is situated in relation to the radiator heights is immaterial. I would guess in domestic installations, the majority of boilers are below the height of the upstairs radiators.
The only consideration with pressure is the minimum required to fill the system. If we imagine the boiler in a basement of a house with heated attic rooms. Given that the highest radiator could be, lets say, 7 metres above the pressure gauge on the boiler and each metre head of water is about 0.1 bar, the gauge in the boiler would have to read 0.7 bar before the attic radiator would fill.