There has never been a requirement to re-take theoretical knowledge examinations to maintain a licence, only the IR. As in the CAA response earlier in the thread:
To renew your UK ATPL is an administrative process that requires you to apply and pay the associated fees...
In short:
To renew your ATPL, you need to hold a valid Class 1 medical and a valid (EASA) type rating on a multi-pilot aeroplane. The type rating on your HK ATPL will not count directly, although it can be transferred.
To renew your IR, assuming that your HK IR is valid, you need a course completion certificate from an ATO and a pass in the IR proficiency check. Note that the requirement for a course completion certificate does not necessarily imply that you will need any refresher training - that is at the discretion of the Head of Training.