Hi Dave,
Importing from China is much the same as importing from most other non EU countries. Firstly if it's a personal import then the decision to use a UK importer or Chinese exporter would be based on price and how you intend to deal with customs. Both DHL and FedEx have extensive networks in China and therefore offer good prices if arranged from the UK, they also have their own customs clearance in the UK which again makes the process easier to deal with.
Shipping costs are calculated using both "dead weight" and "volumetric weight". Dead weight is the actual weight of the shipment whilst volumetric is a muliplication of the dimensions of the shipment diveded by 6000. For example if you have a box that weighs 5Kg and the dimensions are 50cm x 50cm x 50cm then the volumatric weight is 20.8Kg. Shippers will always charge for the higher volumetric weight if they can.
As for pitfalls, if you would like to give a bit more information on what you're planning on importing then I may be able to advise you. Do the people in China have a shipping agent, if yes do you know who they are?
Cheers
Sean