The furthest south I've ever travelled is London Zoo, and north it's Edinburgh. My general rule with zoos is that if they're far away and particularly hard to get to (like, more than two trains or a journey longer than about 4 hours), I won't prioritise a visit, because journeys that long take a few months to plan out.
However I'll occasionally make exceptions if there's a species there I really want to see. For instance, I'd like to add African elephants to my Zoo Life List at some point in the next couple of years, but the closest Zoo to where I live that has them is the West Midlands Safari Park, which is a five-and-a-half hour train ride away, so as much as I'd love to go there and most likely will, it's not a high priority.