Seems like 8km vis for a x/country is about standard for the comfort zone. I flew with another qualified pilot from Coventry to the Forest of Dean area soon after getting my PPL, and when the vis was getting lower than 10km I was starting to feel uncomfortable. For circuits I'm okay with 3km vis, but not completely happy. I suppose that partly depends on the field you're doing circuits at. Coventry is fairly large, and there are lots of big ground features we use for the circuit pattern. The difficulty comes when the sun is shining through the cloud, and it's like flying through milk. Then it's impossible to spot traffic.