So in reading this between the lines, the Red Guards
could have made that one that last push north and stormed Windsor Castle, the symbolic last redoubt of the British Monarchy... but their leadership instead sent them south to London to deal with the 'real enemy of humanity', the Zeds. In the process, they 'just happened' to attrite their own fighting forces — including many of the hardcore believers who might have pushed for a proper final reckoning with the visibly out-of-touch monarchists — to the point where it was 'easier' to simply accept terms and be folded back into the 'real' United Kingdom.
I mean, I can see the logic — when you just came off of winning a war against the dead, with all the attendant carnage and destruction and misery, why would you dive straight into
another war with the
living? — but considering that most of the 'governments' that emerge in Brooks' narrative are apparently
some flavour of authoritarianism or outright dictatorships (the British 'monarchist' state included), the post-WWZ UK might have emerged from the crisis a far less shitty place if the Red Guards had stayed the course and torn down the last remnants of the old order to rebuild from scratch.
(Then again, that could well have ended with humans getting so absorbed fighting fellow humans that the Zeds ended up finishing off both sides
anyway, at least on the UK mainland. See also: the fratricidal infighting and self-cannibalism on the Republican side that ended up handing the Spanish Civil War to Franco's Nationalists....)
[All OOC, of course — I have no idea how I'd handle a Zed apocalypse, especially one that seems to have occurred mostly because multiple direct Acts of
Writer God
forced it to 'succeed', so I wouldn't know where to
start RP'ing a 'Z-sona'.]