"The Jersey Doomsday Book was compiled in 1331. Syvret and Stevens (1998) suggest that there were at least 2,000 houses, and with an average of six persons to a house, at least 12,000 people"
"The first thing that would surprise us about those towns would be their small size. At the time of the Domesday Book in 1086 London had a population of about 18,000. By the 14th century it rose to about 45,000. Other towns were much smaller. York may have had a population of about 13,000 by 1400"
I was going to put something here about the population of iceland, but it was too insigificant for anyone to actively measure at the start of EUIV's timeline.
As I said before, the process for Paradox to add countries and/or provinces can only be an arbitrary one. If it were done by population, Jersey would actually be around somewhere in the top 200 for the timeline start, if it's by wealth, Jersey would likely be even higher especially being a holiday home for active and exiled nobility.
It's disappointing, but all you can do is constantly pester and/or ask for provinces and countries to be included if they aren't already. Sadly, custom nations don't fix my problem because the land itself simply doesn't exist. You can at least make a custom nation Iceland and play it.
On the bolded text: either it's from Wikipedia or wiki copied it verbatim (https://en.wikipedia.org/wiki/Jersey's_population). However the next sentence reads:
"Platt (2009) comments that the average death rate in the black death of 1348-49 was 30-40%, and he suggests that by the early 15th century the population may have fallen to 4,000-5,000."
which is in line with your graph:
Regarding the population counts vs London. Would that be the 'urban' population of London vs the entire domain of Jersey? What number of people would you suggest was involved in farming etc. just in the in-game London province, then? Not to forget other settlements covered by that province. And are you suggesting the economic activity per capita on the Jersey islands was in any way comparable to London?