You're OK, you have nothing to worry about. This falls under the 24-hour TWOV (transit WITHOUT visa) regulations. As said above, HK counts as a third country, so the part of your journey that is relevant to China and their regulations, is compliant.

When you check in at HND, get there a bit early and let them know you will be using TWOV privileges at PEK. Japanese staff don't necessarily see a lot of visitors using this status (most passengers on this route are Japanese or Chinese who don't need visas, or foreigners who have them). You may get your situation bumped to a supervisor but tell them to check their TIMATIC database and it will be in there. Make sure to tell them to mark TWOV status on the passenger manifest which gets conveyed to the Chinese on the inbound, sometimes in Japan this falls through the cracks.

I would also show JL the onward flight sequence on the other ticket on KA (PEK-HKG-LHR). While HND staff are not required to interline across separate tickets, since you have a PEK layover well beyond the minimum connection time, they just might do it. This would save you some annoyance at PEK, not having to deal with bag claim. PEK policy is to autoroute international transit passenger bags to the next flight, as long as they are tagged through.

If they refuse to do so and you have to pick up your checked bag at PEK then recheck, not a big deal, you have plenty of time. Use one of the Foreigner desks at Immigration, tell them you are using TWOV, and show your boarding pass from the incoming flight and printout of your onward itinerary to HKG (with your name and ticket number on it), then get stamped in so you can head for the Bag Claim and nonsecure side of terminal. You'll then recheck and get your onward boarding pass (if you weren't able to get this online in advance). While you're on the nonsecure side, get something to eat since the choices are better than once you're back on the intl airside. If your incoming plane is on time, you actually have enough time to go into town via the light rail express, to get some eats. Leaving the airport on TWOV status is completely acceptable, and particularly if you don't have lounge access at PEK, is even advisable.