When I lived in the UK I got a ton of mileage out of trench/raincoats with removable linings. And that would probably be fine for London, but in December it will likely not be warm enough for NYC. (How long are you going to be in New York?)
I'm a frequent traveler and so a light (you might say lazy) packer. When I pack for a trip, I think coat, bag and shoes first--I usually only bring one of each of the first two (not counting a small evening bag/clutch), and typically only two pairs of shoes--a dressy pair and a pair that I'm comfortable walking in. Would something like that work for you?
