Rhubarb isn't native to the UK. Its origins are mysterious (the "barb" is for barbarian) but I think it came over with the Romans. I had a feeling that the Tiber was named after rhubarb, but I'm remembering something wrong there evidently.
Most probably Asian. There are closer Hymalayan species, if I remember correctly
I know it grows well in Zimbabwe at least. My mum used to make a lot of rhubarb crumble. Yum!