I posted this on the visas to others countries forum. Not really sure where to put it. Does anybody have an answer.

 

"My ex wife renewing her UK passport has been told to send her Thai passport with the UK application. I can't think of a single reason why that is necessary. Has some strange rule come into place where dual nationals in the UK need to show their national passport when renewing their UK passport?"

It’s been an option for years and nothing to worry about.

 

This is the advice from the UK Passport Office “For applicants with a foreign passport, HM Passport Office may require you to submit it as part of your renewal application. This is to verify your identity and ensure that there are no discrepancies between the information on your British and foreign passports. It helps prevent fraud and maintains the integrity of the passport issuance process”.