The UK's (or whatever it was called back then) annexation of HK was merely a bi-product of the preceding "century of humiliation" (i.e. Opium Wars and the events leading up to them). For the most part, China has put this matter behind it. However, in recent years, there have been several other diplomatic rifts between China and the UK. I honestly have no idea whether or not there is a connection between these matters and the UK's omission from the Shanghai exception list.