Evidence for compassionate passport fast track

[GrinAndVomit]

Hi,
I have an elderly family member whose health is seriously ailing and lives abroad.
I applied for a new passport so I could see them before they passed but it is now looking more likely to be days rather than months.
I am aware my application can be fast tracked in compassionate grounds but I have no evidence of their health as they are at home with other family members.
Can anyone advise?

Call the passport office. They’re very helpful and can escalate things.

Send a letter with a stamped envelope to return it quicker? This actually worked for me! Although I was asking for it to be sent back to me as it wasn't quite out of date and I needed to travel last minute and said I would just re-apply for the new passport on my return, but they fast tracked it and sent the new one back in my stamped envelope!