Maybe it's the wording but 'the King and his ministers control the Thames Valley west of the ruin that was London'. Charles can not have ministers, and certainly not the PM, in any literal sense The PM isn't even mentioned and IIRC members of the royal family were not to be bunkered with key government ministers. Also, if 'his ministers' is being literal, that puts Charles in charge. If instead it's referring in a deferred way (i.e. when the term 'her majesty's government' is used in a general, not literal, way) then that means something else. RAW it does suggest that Charles is wielding some form of power, in a limited area.
To clear things up, it might be better to reword it and say that Charles has survived, and ministers sharing the bunker at Reading have managed to regain some order in the 'Thames Valley west of the ruin that was London'. I'd also add a line noting that possibly the PM has survived and is in a bunker at X, whilst other surviving bunkers around the country are trying to bring some semblance of organisation and order to their local areas.