Hi Kun Kantara,
Generally in the UK for GCSE's you will study 5 hours per day, 25 hours per week, of actual teaching time within school and are generally expected to do 1-2 hours of homework per week per subject (most don't do nearly this amount). You are already exceeding this (at 5h50/day), and the fact that you are studying for 2 hours each evening puts you in the top 10% of GCSE students (a lot of which won't do nearly this amount of work).
I wouldn't worry about not being able to complete the courses.
You have already done work in most of the subjects you will be taking, and probably don't need any further "catch-up"... but feel free to check the syllabus for each subject if you're worried. Most schools will do AQA for the sciences and Edexcel for Maths.
If I were you, I would get a copy of the syllabus you have been working to in Hong Kong. You can then check this against whichever Exam Board your new school uses for each of its subjects. Google is your friend... there are tonnes of past papers around for you to check your learning, and see where you need extra practice as you go.
Your main probems are going to be Chinese and Economics at GCSE, because not every school will offer these subjects (and I don't what C&SD is, sorry), and Biology... some schools will allow you to take only 2 sciences, but most will give you the option between Triple Award (1 GCSE per Science, 3 GCSEs total) or Double Award (2 GCSEs with 2/3 of the content for each Science from Triple Award (If that makes sense?)).
I would check this with whichever school you end up at, as most won't allow you to simply take an exam in subjects they don't offer. This is because they don't know your level of learning, can't assess your level of learning because they don't have the teachers to do so, and don't want you to get a bad grade that reflects badly on them and their Exam Tables, which is used to judge them by OFSTED.
But you can take them privately, for a large fee, somewhere between £200-£400 if I remember correctly.
If you haven't covered any Biology for awhile then this is where I would suggest you start. Use resources like BBC Bitesize and the AQA syllabus and past papers to give you an understanding of what to expect, and make sure that you have a good grounding in Key Stage 3 (KS3) material (learned from Year 7 to Year 9) to help you understand the more complicated aspects at GCSE.