If you've ever played Crusader Kings II, there is the concept of a personal demense, the land that a ruler can reasonably personally profit from without distributing titles of nobility and vassalage. In ACKs, this is the personal domain of the ruler, which is generally going to cap out at 16 6-mile civilized hexes, plus whatever the ruler can grow the city within their personal domain to. Everything else is assumed to be folded up under the layers of vassalage beneath them. If you get to the level of vassalage of Kingdom with principalities, the portions of the realm not under direct control of the ruler are going to be dramatically smaller.
Now, it's true in the past that the King of England is often also in possession of a principality and a duchy, but this doesn't have to be the case. The aforementioned CK2 has a much more granular way of handling titles of nobility that determines when a ruler holds too many titles, but in ACKs you can still slightly achieve this through some additional rules on vassalage that have been added since the game came out.
The default assumption is that you'll have 4 henchmen, plus or minus your charisma bonus and a few other proficiencies. These are to be reserved for the largest swaths of land you'll be parceling out. Assuming you're a very charismatic ruler, you might have as many as 7 henchmen, and if you're the king of england you might use 3 of them for the principalities you do not personally control. These princes presumably have dukes and counts and barons somewhere beneath them.
But you are also a prince in addition to a king, which means that you have dukes beneath you just like your princes do. Perhaps these dukes will use up another 3 of your vassal slots, drawn from henchman who came along later in your adventuring career and maybe aren't quite as loyal, but still rather loyal.
Finally, you have at most 1 space for a loyal vassal left, which as a Duke in addition to a prince and king, you might use for a count vassal. However, you do not need to stop at 7, or possibly more because there are some proficiencies that will allow more. If you wish to have more vassals than this, you can, but they are more limited. I can't remember the rules off the top of my head, but it amounts to a -2 on loyalty and never giving a free monthly duty without first recieving a favor, which your loyal vassals would.
Even without an 18 charisma, you could make heavy use of these less loyal vassals to grow the number of direct reports you have, but it stands to reason that only the most charismatic and capable rulers of the eras will be able to manage the challenges of so many direct vassals.
To answer your last question: you should almost certainly assume that London counts for all of those cities being proscribed, although it stands to reason there are other, smaller cities in each case, but each of them is being passed on as a capital at a given layer in favor of london.