There are no official OW baggage privileges across the alliance - you officially only get them on the airline where you have status. And, as you realise, the two tickets means that you don't get the "Americas" piece-based allowance on the LHR-HKG-LHR.

So if they do give you more than 30kg for free then they are doing you a favour. I would recommend getting it down as low as possible by loading the heavy stuff in your carry on. Up to 35kg I think you'll be fine, but 46kg is pushing it more than a bit.