My view, for what it's worth, is if you do a powerflush, you do each one individually. There may be some confusion in the two statements about powerflushing in that first all valves are opened and the whole system is stirred up. Next all radiators have descaling chemical pushed through. Then all radiators but one are shut, and that radiator is flushed. This is repeated for each of the radiators in turn. Then the radiators are flushed with clean water, both directions, until the water runs clear, and again repeated for all radiators. Once this is finished, the acidity is checked and adjusted before finally adding corrosion inhibitor.
It may be your system is so clean, that a full flush is pointless. I have had one situation where this happened, and instead of a £350 exercise, I did it for £150 as it didn't justify any more.
Why are you replacing the boiler? You should have a good reason for this, and not just be told by some ******** that "it's on it's last legs", "it's not economic to repair", "it's inefficient".
I would even seriously look at oil as well as gas. I have a customer with a large 4 bed detatched house who used around £240 worth of oil last year, and they don't have the house cold. Oil boilers are inherently simpler and thus much more reliable than gas (look at the number of oil faults on the forum compared with gas, and even correct for the difference in total numbers in existence, and they're still better.
Be very careful about a combi, there is lots to read on the forum, and bear in mind they are NOT more efficient than other boilers and neither are condensing boilers in most cases. Despite what the government would have you believe. You also need an adequate water supply, has the installer checked it and told you the pressure and flow rate?