Jewlz, M&S carries egg pasta that's pretty similar to US egg noodles, just a bit bigger. Can't remember what the shape is called, though.
I actually like British soured cream and creme fraiche better than US sour cream. They don't have as many preservatives, gums, thickeners, etc. The half-fat creme fraiche is quite nice and works well in stroganoff.
Oddly enough, I never had any trouble finding buttermilk in Belfast, but it seems to be a bit harder in Leeds. It does exist, though. As chary said, it's usually with the cream.