Interpretersky

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If you fly to HK by Cathay Pacific you can book through to Xiamen although they will have to change flights as HK to Xiamen is by Dragon Air which belongs to Cathay. 

That is quite cheap when it is combined with the London - HK flight and they would offer the same with Canada - Hong Kong flights. You could stop off in HK for a few days before going on to Xiamen for the same cost but you’d better ask.