I live in England and yep, there is a sun up there somewhere...

As I understand it (and maybe I don't) the atmosphere acts as a big UV filter and the more atmosphere the sun's rays have to penetrate the less UV it contains. Consequently at higher latitudes when the sun never gets very high in the sky the rays have to penetrate a lot more atmosphere than when the sun is overhead in mid summer at the equator. This means that at the moment it would be impossible for me to get a sun tan (and my cyanotypes take ages) but I can walk about in the day time without bumping into things ;-)

When I went on a summer holiday to the Yosemite I over-exposed everything. Using my own calculated film speed and trusty meter I failed to allow for the fact that the brightest light at 9, 000+ feet in clear air that far south the light must have been much more actinic than anything I was used to...

I suppose it will also depend on the film and equipment you use. Maybe a lens with a lot of glass in it and a uv filter on the front might not show much difference compared to an unfiltered, uncoated triplet?