Running chemicals through the system with all radiators removed?

In addition to the previous posters reply I would advise you to remove all of the redundant radiators and bridge the existing valves with speed fit pipe (cheaper than copper) prior to power flushing the entire heating circuit. No point in cleaning the existing radiator's if they are being replaced! And this will reduce the power flushing regime time scale too.

You'll need to keep the old radiators connected. If you remove them, the water will not flow through the full length of your pipework. If you have a combi boiler then this will no doubt be a sealed system which has no tank in the loft. You can add a cleaning chemical through the filling loop at or near the boiler. Run the system for a few days with the chemical added, then drain off and refill. The best solution is to employ a heating engineer to power flush the system. This cleans your old pipework much more efficiently than adding the chemicals yourself. When you've finished remember to ad corrosion inhibitor once the system is refilled. Otherwise internal corrosion will return and will damage your new radiators.