I currently live in Manchester where an entire district of the city centre is called Piccadilly, I know from other towns and cities that this is not an uncommon name, where does it come from? Is it named for a battle or a general?
A piccadill is 'a large broad collar'.
Piccadilly Road in London, probably the most famous example, is named after a tailor's house that was built there:
The origin of the word Piccadilly as a placename has been the subject of antiquarian discussion since 1656, when Thomas Blount first propounded two theories which still command widespread assent. A 'pickadil', said Blount, was 'the round hem, or the several divisions set together about the skirt of a Garment; or other thing; also a Kinde of stiff collar, made in fashion of a Band. Hence perhaps that famous Ordinary near St. James called Pickadilly, took denomination; because it was then the outmost or skirt house of the Suburbs that way. Others say it took name from this, that one Higgins, a Tailor, who built it, got most of his Estate by Pickadilles, which in the last age were much worn in England.' Most subsequent debate stems from this pronouncement. Successive historians down to the present time have discovered fresh crumbs of information about the early history of Piccadilly, and a fairly complete picture can now be drawn. Yet elements of uncertainty still remain, and will probably always remain, to challenge further research.
[...]
The present writer therefore concludes that Piccadilly Hall was a derisive nickname bestowed upon the house built by Robert Baker, a tailor who had made much of his fortune by the sale of piccadillies; and that the situation of the house 'then the outmost or skirt house of the Suburbs that way', may well have given added point to the name.
I would speculate that many other places named 'piccadilly' got their names in similar ways, but I have no source to back this up.