Aside from little bits like Berwick, Oswestry etc?
Cornwall? Cumbria? Northumberland?
define 'part of england'. cause i'd suspect there's diffeent answers depending whether you're looking at last bit conqureed, or considered a separate constituent nation that was absorbed into england, their own language or whose people did not consider themselves English. IDK the answers, but i do know cornish hsitory. sort of. as much as you can seperate history from mythology.
AFAIK, althastan didn't conquer cornwall, but he did set the boundary at the tamar and evict the cornish from exeter. the king of cornwall paid tribute so cornwall wasn't invaded. Actual invasion didn't happen until 1066, but William granted lands in Cornwall to his Breton followers and at that time, language and customs of the cornish and bretons were very similar. there's some descriptions I've read that call that a 'return' but the people that refer to it as that also come up with some myths that make my head repeatedly hit my desk.
I think that elizabethan maps were the past to show Cornwall as a seperate area to England. and elixabethans did so less than in previous eras. the general belief i see is that there was an intentional destruction of cornish sepeateness because of the rebellions.
Seperate language is also contentious, and although the evidence for language survival into the 20th century is hearsay, there's a hell of a lot of hearsay about from people without apparent agendas, some of which i've heard and passed on to those who collect such snippets. i personally believe that there were people with some use of the language that at the least overlapped the revival era and the reports of the death of the cornish language are mainly dependent on long and complex discussions about how and where language is used in order for it to be a living language.
Cornish identity never went away. i've seen descriptions of cornwall having an identity akin to that of the Irish by an english unionist in the 1880s.
legal status - no justification for still calling cornwall a seperate nation. if you're terribly interested in that, then I'll dig out links to the latest research, incidently from the bloke who really, really pissed off 'the establishment' by winning his FOI case against the duchy.
Oh, and the duchy either has the constitutional status of the crown in cornwall, or is a private estate, depending on which is more convenient to the duchy at any given time and place and is subject to change without notice and cannot be questioned in parliament.