舊款勞斯萊斯維修由大昌行負責？

The answer is already in your "p.s.": pre-BMW RRs were manufactured together with Bentley.  That plant is located at Crewe, mid England.  So that means Crewe is having all the tooling and parts as she is still supporting pre-VW Bentleys.  So simply, it is more effective to have old RRs service with workshop with direct connection with Crewe, which is DCH.

This situation is not limited to HK.  Especially in the UK, Bentley dealers are reminding old RRs to serivce with them instead of BMW dealers.

無錯,我嘗試過幫一部88年既Silver Spirit訂番套原廠車匙,
打左去BMW問,
佢話Silver Spirit要同番大昌訂,
打去大昌問,又真係佢地先報到價比我