My Grandmother was born in Poland in 1912 and moved to the UK when she was 2. The only evidence I have is her passport which states her original name and that she was born in Warsaw with the date. I have no birth certificate etc. Obviously this is before 1920.
Citizenship - grandmother who came to UK in 1914
Well her passport is Russian. Somehow I doubt you look for Russian citizenship.
She is on the passport of her husband (my Grandfather). It is a British passport. His family also came from Poland but some decades earlier.
She is on the passport of her husband
I'm not sure how it worked those days but I'm pretty sure that from a legal point of view her citizenship was Russian.