I guess this comes under ancestors who moved around before census eras, but you still have to find the documents to show they did move around. And often if you cannot find the evidence, it can be hard to determine, even if they had a locative surname such as the Norfolk name Cattermole in Birmingham in the late 1700s for example.
As was customary, lots of cities had the strays, the ones who moved there looking for work, especially London. For instance looking through St Margaret Westminster records for the 1700s I found surnames like Horsfall (a Northern surname) among several other locative surnames.
And interestingly my ancestor was a William Inkpen who wed in Oxford in 1765. He was a college servant, and later a publican before dying in 1769. His wife Jane (Nee Gater, later Coles before becoming Inkpen in 1765) was from Burford, a large Oxfordshire village near the Oxon/Gloucs border. They wed by licence, and the original licence says William Inkpen was a servant, bachelor, aged over 21, sponsor was Richard Baylis. His marital status not given in actual marriage register for St Peter In The East, Oxford. Witnesses to actual marriage was Richard and Mary Baylis. Now the surname Inkpen may have very ancient origins in Inkpen, Berkshire but by the 1700s Inkpen was a Sussex, Kent or Dorset surname. No known Inkpen baptisms, marriages or burials in Oxfordshire prior to 1765. William died in 1769, no age given at death. They had 2 children, James and William. James seems to be from Jane's side as her father was James Gater.
The above example is food for thought, that William Inkpen was probably from Kent, Sussex or Dorset. I have looked for any further info on him and his origins but to no avail. No poor law records for Oxford mention any Inkpen's.
I know Oxford was a city that also had a lot of people move there to study, but probably also to work in the colleges.
Also In have a weaver ancestor called Dennis Helsdon who wed in London in 1784. It did not take me long to find out he was the same Dennis Helsdon born in 1756 in Norwich, Norfolk. And his wife was Susan Helsdon, nee Fradine, a Huguenot.