This is due to the politics of the UK. in the UK, Scotland, Wales, and Northern Ireland are first class citizens, along with England.

It is the United Kingdom of Great Britain (England, Scotland, Wales) and Northern Ireland.

Northumbria, however, does not exist anymore, its now a bunch of English counties, and the southern half of Scotland (south of the Firth of Forth).

Without more details on which part of Northumbria is meant it's impossible to know if Enlgand should be included, it may also be wrong to include England if its only a small part and its most / all of Scotland, Wales, and Northen Ireland

The author may be getting confused with the similarly named modern county of Northumberland. You'd be surprised how often the border region between England and Scotland is confused, misplaced, or forgotten about.

[Source: I'm from Northumberland, technically Tyne and Wear, but thats splitting an already split hair]

Edited for clarity