Hi I'm traveling to Bangkok, and going to a jewellery fair..

There I will be buying gold and silver jewellery.. I'm then going to china for a few days and then back to Bangkok... From there I will then return to the UK... Can anybody advise me as of how and what to do as to declaring in and out??? I don't want to buy or sell in china... But want to keep my belongings with me... Do I have pay duty all the time??? Or do I just tell them and declare it and they let me through and explain???? Please help someone