UK first passport issues with expired foreign passport

Hi everyone, please help (currently internally panicking)

Countries not listed due to anonymity
Country A: my birth country
Country B: commonwealth country

I have applied for my first British passport and was contacted to send in my other foreign passport which came to light in their checks.

I was born in Country A but this country does not recognise dual citizenship so when my family immigrated to a commonwealth country (let's call this Country B) when I was a child, I became a citizen of Country B and so my citizenship in Country A was automatically cancelled. My parents also changed my name after I immigrated and I've declared my previous name on my application.

I am at a total loss of what to do as I don't have a valid passport to Country A as I am no longer recognised as their citizen, and I don't have my expired passport with me as it's been >20 years. I've contacted extended family to see if they can try digging this up.

I've tried explaining this over the phone and they said they will pass a message on to the examiners/reviewers at HMPO.

Can I be refused a British passport for this reason? Is there anything else I can do? Really panicking here as I may need to travel in the next months.

You are British and can apply for a passport. Having a foreign passport is not a legal requirement.
Why not send passport B and include a statement that you don't have A anymore?

I suggest sending a letter that explains the situation with your application reference number to the office that is looking at your application. Advisors on the phone may not relay your message to the case workers.