It would need a PoD or possibly two, but it isn't totally unreasonable to have Northumbria survive as a quasi independent vassal, both part of, and not part of England proper, longer than it did in OTL. Whether that can last to 1200 without being subsumed into England is another question.
Casting a huge butterfly net over the intervening years, we then look to a revival of Northumbrian identity in the 1800s. Very difficult, but not impossible. The main difficulties are differentiation and the first Northumbrian identity. The identity, or at least the perception of the identity, of Northumbrians in 1200 and 1800 must be different enough to both English and Scottish to be worthy of revival. Maybe a more Highland/Celtic focussed Scotland could help achieve half of that? If Northumbria is not just part of a continuum, linguistically speaking, then that might help foster a feeling of identity different to both Southern England and that of Scotland.
Secondly, and more difficult to overcome: in late tenth and early eleventh century Northumbria, there's essentially two identities. Yorkshire is Anglo-Norse, with loyalties and ties to Scandinavia that last beyond the fall of Erik Bloodaxe and into the eleventh century. North of that, Durham and Northumberland is more Old Northumbrian (in some senses of the word), dominated by the Bishops of Durham, and Earls of Bamburgh. Uniting those two identities into one overarching 'Northumbrian' is needed, or else it'll never exist sufficiently to re-emerge.