You mean do your Part 2 in America? A Part 3 is getting the registration. You cannot get British registration very easily with an American Pt 2. Its very difficult. What do you need for Hong Kong? You could complete your Part 1 in the UK and come to America and get a 3-year Masters of Architecture. That is a pathway that people take. 

Hi James, what makes you say getting part 3 with a US part 2 is hard, have you done it? Or know anyone who has. That's currently the route I'm taking, so I would appreciate any information you have

There is not Part II in the US. The education systems are different, as is the route to registration. There is no reciprocity between the two countries at the moment. If you want to study in the US you should be looking at a Masters Course as you undergraduate is already in architecture. You can then go through 3 years IDP  working and sit the American Registration exam. You will then NOT be able to call yourself an Architect in the UK, without retrospectively sitting the ARB Part II exam and Part III exam. If think you will end up living in the UK / rest of the world ( apart from the US) for the rest of your life then stick with the British System. It is the gold standard, as the RIBA Validation is based on quality.  Part III is an exceptional professional course.

The Slide show below goes some way to explain the differences in the  two systems.

http://www.slideshare.net/DebbieBentley1/similarities-and-differences-in-the-university-education-for-architects