I guess the Yorkshire countryside is probably depopulating like many rural areas are, so they probably have extra capacity in the water lines. In London, decommissioned gas lines used to be sold to broadband companies to avoid digging up the streets. The difference here though might be that every waterline repair will probably lead to an internet outtage and vice versa. Since this is countryside and they probably wouldn't get the service otherwise, that's probably a trade off their happy to make.