What are you painting. Is it a single item or does it need to match other wagons?
Does the colour have a BS number. or is it just British Rail Blue?
If its a stand alone, I would get some Vallejo model color Prussian blue, both the dark and the light and some PRU blue. See which is closest and mix them to get the best result. Dont forget to let a test swatch dry befor you commit. Been there, done that. Mixed a perfect match to some thing once, panted the model and went out. The colour once dry was quite different. Oh and make sure you mix enough in one go... lol