I find it very fascinating to compare the different density numbers. (especially when you take a lot at the countries behind the numbers and what they do and do not have in common)
The actual number would be about 10.000/km² if you go by 5 people
Just did the math(s):If you take out northern ireland (so both the population and the area) you would be at 282 instead of 279, so that makes hardly any difference at all
The Netherlands is actually at 507 as of 2019. The UK as a whole is at 279 and england is near 430. So not comparible with the netherlands at all. England is closer in density to, well nothing is near actually, it is sort of between burundi and haiti (449 and 409)
Roughly England has 60% of the area and 85% of the population, so the rest of great britain does not come into play.