Nationalism is incompatible with left-wing politics
Except that if you read the BNP's manifesto, it's overtly left-wing on a host of issues, which is one of the reasons they're getting so much play. Economically, they're easily the most left-wing party (anti-neo liberal, pro nationalisation, anti-globalisation etc). What they don't do so much is internationalism - which is a big part of left thinking but not the only part.
It's a bit hard to do that when you're part of what was once the largest empire in the world
No it isn't, the US spend most of their time playing the beleaguered underdog. Britain went into the first world war pretending it was the victim of German imperialism ffs.
However England is a nation
Your 'nation' is the collective fantasy that a rich southern businessman has something in common with a dirt-poor northerner, through a shared 'heritage' which amounts to not a lot more than brickwork/paintings/music created by people you've never met and some musings by military historians.
England' is a piece of geography, lorded over by an avaricious and jealous state enforcing the interests of a truly international ruling class. I'm not hostile to England, I'm hostile to people irrationally accepting base propaganda about the existence of a united 'country' whose most treasured monuments were put together by warring clans to whom an overall nationality would have been laughable.
Citizen of the Earth, Subject of the Crown.