I've attended a talk at British Council last week and the speaker introduced a UK web site www.thebookpeople.com which offers English books at a very attractive price. The limitation is it only delivers to UK address.
To place order, you have to ask somebody in UK to send the books to HK for you.
Here are some good stuff I found for your reference:
Magic Key of 12 books at 9.99
Usborne Farmyard Tales - 20 books at £12.99
Maths Start - 6 Books by Stuart J. Murphy at £6.99
Active Readers (age 4-6) - 10 books at £7.99