Would campaigning for a county independence in England be considered a crime? Or is it totally legal?
Yes, it is legal to call for secession. This really a legal question rather than a political one. The definition of high treason in the UK is narrow, restricted to waging war, giving aid to the enemy in war, and a few other more specific acts that aren't relevant to secession. Other nations do have more draconian laws, allowing them to prosecute people peacefully advocating secession, but this is outside the scope of the question.
The other relevant offence in England and Wales was seditious libel which is no longer a specific offence; sedition as an offence was abolished in Scotland in 2011. If you support separatist terrorists in any way, you may fall foul of anti-terrorist law, but in the absence of terrorism, you are free to call for independence of any part of the nation you want - just as there has never been any question that it's now legal to call for Scottish independence.
Finally, it was formerly illegal to advocate for the abolition of the monarchy; even if this was relevant to calling for some part to become independent and a republic, which is uncertain, it is no longer an offence to call peacefully for abolition of the monarchy. The Treason Felony Act 1848 has more recently been interpreted such that peaceful calls are allowed in accordance with the 1998 Human Rights Act in the name of freedom of speech.
Yes, this is completely legal - several political parties campaigning for independence for counties and other regional localities already exist, for example the Essex Independence party, Londependence, and the Northern Independence Party. Mebyon Kernow is another example that advocates not for Cornish independence, but for the county to be put on the same footing as Scotland and Wales in the United Kingdom.
These parties have not had incredible success - the candidate supported by the Northern Independence Party in the 2021 Hartlepool by-election received just 0.8% of the vote, and the Londependence party, although set up in the wake of a petition signed by 180,000 people calling on the Mayor of London to declare independence from the UK after the 2016 Brexit vote, only attained 0.2% of the vote in the May 2021 London Assembly elections. Mebyon Kernow currently holds five of the eighty-seven seats on the Cornwall Council.
Nevertheless, it remains legal to advocate or campaign for a county's independence.