Might be easier to do in Hong Kong.
Non-UK residents and those temporarily outside the UK
If you do not live in the UK, you can apply in person to renew your passport while visiting the UK.
IPS cannot accept applications from overseas. If you would like to renew your British Passport while outside the UK, you will need to contact your local embassy, high commission or consulate.
Renewing your passport in the UK if you live abroad
You can renew, amend or replace your passport whilst you’re visiting the UK. You should apply to the Identity and Passport Service.
You’ll need to make an appointment, provide a verifiable residential address, and be able to attend any interviews as required by IPS. You’ll have to be available at this address so you can sign for the passport. Please note that IPS do not accept applications by post or email from abroad.
It seems you need to make a visit to them,rather then renew by post.