For your question, we would like to answer as follow:
When you renew your driving licence, your UK passport is needed because your driving licence application processing will be faster:
In case you are 70 years or over, your UK passport is not always needed. However, your application process will be faster if you provide us with your passport.
In case you exchange your paper driving licence for a photocard licence, sometimes your application is accepted but sometimes it is not. Please provide your passport number to make sure your application is accepted.
In case you renew your driving licence(10 years replacement) or replace a lost, stolen, damaged or destroyed driving licence, your passport number is compulsory. If you do not provide your passport number, your application cannot be processed.