I'm wondering how so many things in Manchester, the main rail station, a turning basin, a street, and a park all got this London-origin place name?
The theory behind the name Piccadilly is that it comes from "pickadil" was a lace attachment to a collar popular in Elizabethan times and that the places in London and Manchester were named because this is where they were sold. This was put forward in the 17th century but was conjecture then.
I'm finding the name Piccadilly in Manchester in 1801 https://upload.wikimedia.org/wikipedia/commons/0/09/Map_of_Manchester_1801.PNG
which predates the names of the turning basin, park, and railway station. So it's been used there a long time.
I think that's right, Emma, I noticed that both Piccadillys lack a "follow up" word (lane, street, avenue, etc) which I think is pretty conclusive that one is named after the other. And this area of Manchester was brand new in 1801, right on the edge of the city. Note that the current London Rd continuing southeast of Piccadilly used to be called Shooters Brow! And I can't find any source online older than 1800 that is not London related.
If anyone is interested, this is what lead me to Piccadilly in Manchester, looking up the location of Salford. http://www.podbean.com/media/share/dir-iy37w-19c6624
Interesting use of shooter, but not as exciting.
Seeing a Pickadilly on this London map of 1741 https://upload.wikimedia.org/wikipedia/commons/3/3a/Rocque%27s_Map_of_London_1741-5.jpg
Also a Black Fryers Bridge, makes me think of cast iron pans.
Manchester Piccadilly Station itself was called Manchester London Road until 1960.
Digging around a bit Piccadilly the street was previously called variously Market Street Lane and Lever's Row. The name was changed in 1812 - possibly because there was and still is a Lever Street.
Many people say, I don't know about this, but many people say, believe me, that "dilly" used to be slang for "pimple", so "piccadilly" referred to picking at pimples, and the places where adolescent youth would gather would be known as piccadilly squares. It's right there on the internet.