I know there are trading companies in UK. They get orders in UK and they find suppliers in China and place orders with China suppliers. How does a British trading company get orders? I mean, why doesn't a British customer work with a Chinese trading company rather than a British trading company since a Chinese trading company can always offer a better price than a British trading company?
As someone who've worked in HK and China I can give you the pros and cons
Local companies are bound by domestic laws, such as TradePractices Act and warranties. So the price a Chinese firm may quote is ex-factory or FOB. As others point out once customs, insurance and freight is factored in (see book Poorly Made in China on short-fibre packaging which falls apart) the price may escalate.
HK companies have recognised that with Alibaba, foreigners can go straight to their suppliers so they focus on logistics, supply chain coordination and a whole of international compliance issues that a pure manufacturer doesn't care about. For ExIm, the difference may be marginal but when you are trying to coordinate across multiple components and offices, having a single point of contact simplifies matters.
The book Poorly Made in China makes the point that the first item is usually precisely to specs but that quality fades over time as margins are cut. The example author gives are pockets whose depth are just a couple centimetres too short to fully insert his western fingers. Save a few cents at cost of pissing off your customer(s)? Not to mention subsuppliers which you have no control over such as illegal abestos brake linings in chinese made imported cars?
There are times where there is no choice except to deal with chinese firms. Example is limited export quotas or a JV may make it conditional to partner up. Fortunately the competition is forcing them to smarten up their act (though still uneven) so taking the time to develop long-term relationships can be worthwhile if you believe your interests are aligned ... just be careful of the old saying, "same bed different dreams" if your supplier starts treating you like a competitor.