Hot water Cast iron radiator

Same as any other panel rad really - the typical inlet at one side at the bottom, outlet at the other side at the bottom, then a bleeding vent and blanking plug for the other two connections will be fine.

Other arrangements with flow and return on opposite sides will be nearly as good. Flow and return on the same side is usually the least good. However it does depend on the shape of the rad a bit - it would matter less on a tall narrow rad.

In my old Junior school in one place the pipe came through the ceiling and went into the top them the run continued all at the bottom of the radiators. I never bothered to figure out which way the flow went, but I'd have thought from upstairs to down, not the other way around!

Even at 7 that would seem silly. Brian

You are (probably correctly) assuming a pumped system. Many older installations with cast-iron radiators would have been large bore, gravity systems and so would want hot into the top at one side and cooler out at the bottom at the other side.