That is a real find!
I have been looking here in Britain for something similar for years, without success. As I understand sporting rifles in .303 calibre were only made for a short time and then the UK Government banned the use of military calibres in the empire, and so the .450s became the 470s, and some Lee Speeds were chambered for an 8mm round.

I have had two of the H&H rook rifles, but a break barrel .303 would be the cherry on the cake!

HB