As I understand the song in total - and it's inapporpriate to quote any song out of context because it misses the overall point (I am not suggesting that this is what you have done 4get, I know you are quoting for your Question) -
the song is a\ vitriolic protest against governments who use young men to populate invading armies, with attendent casualties and fatalities, as we all know far too well.
The use of the phrase 'white ni**er' carefully juxtaposes the image of the 'ni**ger' as a disposable slave-based cannon-fodder grunt, with the thought that in the case of Oliver (Cromwell)'s army and followng armies, the disposable canon-fodder grunt is a white man, but non the less dosposable for that - leaving his widow behind.
I assume that because the term is used to make a social and political point through the medium of song, it has been left uncensored by the notoriously twitchy BBC manderins - or, more likely, they just think it is a nice pop song and have never bothered to listen closely to it at any time - as they appear to have done with Lou Reed's 'Walk On The Wild Side' with its clear storyline about the Warhole Factory stars, and attendent drug use, sexual practices, and blurred orientation.
Where was Mike Read when the needed him?