Returning direct from Beijing to London might well do it - as mentioned above it depends if the agent/fares person wants to see it as LON-NAN and return or LON-TFU and return. From what the agents have told you, it does sound like they're considering it LON-TFU with an indirect routing via NAN. No idea if there's an official definition on that though. Back in 2019 we did AMS-MAD-LAX-NRT;KIX-HKG-AMS [stopovers in LAX and Japan] - when I phoned up to add the LAX-NRT as it was released the agent in the Indian call centre tried to reject it it as "not a sensible routing" or something like that. The UK agents had no issue with it the next day though, and fortunately the F seats were still there. I think the "there and back again" "rule" is more recent than that though, so it doesn't help you much here because AFAIK RTW itineraries were acceptable back then.

I know it's all the rage on here to go to any length to avoid APD, but given that that's less than £200 I wouldn't see it as worth the effort on an award booking if you're actually based in London. Personally I doubt I'd start this kind of ticket without an overnight in the ex-EU departure point, so with hotel and LON-DUB flight cost you've spent that £200 anyway... Different calculation if surcharges are completely different from the ex-EU though.