Rosie, it says on the guidelines

• hold a British, Irish or other EU, US, or Commonwealth passport (in that order of preference) which has not run out. We will be able to process applications more quickly where the countersignatory has a British or Irish passport.

If you cannot comply with the countersignatory requirements your application may be delayed. You must provide a full explanation when you get to the ‘Additional information’ section of the online form and we will contact you to agree a way forward.

https://www.gov.uk/government/upload...otes_10.16.pdf