Citizenship - grandmother who came to UK in 1914

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.

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.

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.