HSBC, Hong Kong Shanghai Bank Corp, was formed in Hong Kong with the funds from Shanghai escaping the communist control. It is one of the top five banks in the world depending on how you rank them.
It has been profitable due to the growing Hong Kong economy (due to its proximity to China) and the overseas Chinese (esp. in S.E. Asia where the government is not too stable ) deposits. The reason they do not want to move back to Hong Kong is they're are mainland China will take over Hong Kong and void all the previous agreements.
As in most global companies, they should seek the best tax shelter and regulations. Britain may not be one but it is prestigious. Their current risk is HK's housing market which is slowing down.