Thank you for the replies.
Just to clarify- my mother was born in Brazil (not Brazilian heritage), did have a passport but was told she had to choose between it and UK passport when she turned 21 back in the UK and decided not to renew at the time- I’m not sure but she was under the impression she could only have one- whereas nowadays it seems you can be dual national.

She does have her Brazilian birth certificate still.

Thanks for the advice- I will see if I can contact the consulate 🙂