My understanding is that the extra arrival into Asia is allowed on the way from SWP to Europe as long as you don't have a stopover in Asia. There's no requirement that the transfer be "direct". So in your plan as long as you don't stay in NRT or HKG for 24 hours, your itinerary should be legal. But if you've already used up your 4 free segments you'll have to pay extra for the HKG-NRT segment. So unless you have some reason to be in NRT for <24 hours, you may as well just go HKG-LHR.