Medieval spellings originally suggested for inclusion by FONS:- Wethell, Wethiell, Wethyell, Withell, Withiell, Withyell, Wothell, Wothiell, Wothyell, Wythell, Wythiell, Wythyell. Others found are Withull, Withhille, Withele.
Medieval spelling of bynames varied between localities. For example, an assize in Oxford mentions Hugo de Withhille in 1207 and Hugh de Withull a knight in 1223 while Reynold de Wythele who held land by the King in 1311 at Wythele 'beyond Trent' may have been from the tiny village of Witherly near the border of Leicestershire. The John Wythiell, Thomas Wythiale and Thomas Wythyale who were goldsmiths in London and Bodmin were possibly from the one family.
Post 1600:- Withiel, Withiell, Withill, Withell, Withel.
The census returns for UK between 1841 and 1901 reflect a major shift in the distribution of the surname, particularly with regard to Cornwall. During these years the Cornish migration was much more complex, with families moving to Wales Devon, London and even Scotland, as well as Canada, Australia and New Zealand. Although during this time one Yorkshire family had moved to Canada and several families had settled in New Zealand, the numbers in Yorkshire appear to have been reasonably stable.
Allowing for some misspelling of entries for known family members from both Cornwall and Yorkshire, the 1881 census recorded 141 occurrences with the variants WITHELL. WITHEL, WITHILL and WITHIEL. Cornwall had 16, Devon 14, Durham 8, Essex 5, Hampshire 1, Middlesex 7, Norfolk 2, Surrey 4, Yorkshire 54, Glamorgan 27 and Scotland 2, with 1 in the Navy. We have also positively identified family members who had been listed as Withill, Withall and Withett.
There have been later waves of migration particularly after WW II when several families of Withells of Cornish descent moved to Australia.
The 2002 Electoral Roll for UK lists 97 persons with the surname Withell, but no Withells currently resident in Cornwall. There were 53 Withells of voting age in Australia in 2000. I don't have current Electoral roll listings for Canada and New Zealand
It seems likely that the surname WITHILL in Yorkshire may have connections with Withell in the 19th century but further research is needed.
The availability of US Census returns online in digital form has revealed that there were a few Withells born in America in the early 19th century. One family originally came from Germany but it has not been possible to research these names further. However several Withells have been found in US immigration files.