Mike Lyle writes

Yes, most of the UK growers are in Yorks. Passing Wakefield on the
M'way you see fields of it, which for some reason I always find
amusing. It isn't so much about climate or temp, as rhubarb is a cool
temperate crop -- they even grow it in the Faroes -- it really is
mainly about the darkness, so they get pure pink tender stems.

Isn't it also about earliness? Yes, you can grow outside it all over the
UK, but certainly Yorkshire it's more of an early summer crop than a
spring crop, and moving it earlier into spring hits that gap before the
cheap soft fruits arrive.
--
Kay