I've just brewed a bitter using a sachet of Windsor mixed with a bit of Nottingham slurry. I liked the idea of the Windsor esters, but thought the extra attenuation, flocculation, and low-temp fermentation (as I don't force carbonate) of the Nott. might be a good thing. I've just tried it, and I like what I taste.
I'm not a huge fan of the SO-4. A bit rough, I've found.
I've used Saflager in combo with a liquid and it turned out great, but never on its own.
US-56 always works well for APA etc, but next time I'm going to try Nottingham for this style, as it's very clean, ferments dry, but has the added advantage that it sinks like a stone.