For those of you who don't know where Northumbria is, it's part of England. It wasn't always this way: while Northumbrians were English, Northumbria being part of England as opposed to an autonomous kingdom owing homage to English kings took a while to be sorted out. Some people, usually leftists dissatisfied by the defeat of a certain Jeremy Corbyn at the 2019 general election and subsequent ascension to the Labour leadership of moderate social democrat Keir Starmer, want Northumbria to be independent. There are some with different motives for Northumbrian independence, but that's the vast majority of its supporters.
Personally, I do not think Northumbria should be recognised. Northumbrian independence is a ridiculous idea (quite apart from the usual nonsense about it being too small to be a country: North East England has twice the population of Estonia, and nobody is suggesting they should have to rejoin Russia) and the independence its supporters want is independence for the whole of Northern England under the name 'Northumbria', which would lead to the subordination of the Northumbrian nation to the interests of the M62 corridor, not actual Northumbrian independence.