What is the theme of "Men of England"?

The theme of "Men of England" is that the majority of the workforce in England is being exploited. The poem is a call to action to the ordinary worker to stop allowing himself to be exploited and to work for his own profit instead of benefitting someone else. The poem focuses on the conflict between the upper class and the working class. It recognizes the difficulty and backlash that revolution creates, but it warns the working class that if they do not throw off this class oppression, they are working to create their own graves.
The first five stanzas of the poem point out all the inequities that exist in the British social system. The first five stanzas have a series of questions and statements like "Wherefore weave with toil and care / The rich robes your tyrants wear?" (lines 3–4). Each instance in the first five stanzas points out that the working class performs the labor, but the rich profit from it.
The sixth stanza tells the men what to do and gives them a call to action. It says,

Sow seed—but let no tyrant reap:
Find wealth—let no imposter heap:
Weave robes—let not the idle wear:
Forge arms—in your defence to bear (lines 21–24).


The seventh stanza recognizes the difficulty of revolution, in the lines "Why shake the chains ye wrought? Ye see / The steel ye tempered glance on ye" (lines 27–28). The narrator recognizes here that if you shake chains that you yourself made, they might hurt you in the backlash. However, the poem concludes that if the working class does not throw off its oppression, "fair / England be your Sepulchre" (lines 31–32).
https://www.poetryfoundation.org/poems/52304/a-song-men-of-england