Biggest cities in the UK by density

Weird innit? The ONS has Portsmouth as the most crowded city in the country, I believe - certainly the most crowded in the south east, and the figure they give on their website seems to be over 5100 per square kilometre - presumably everywhere else is lower by their measurement. I've also heard it said (although I don't know how true it is) that it's the most crowded city in Europe as it's one of very few on an island. Maybe it depends on where you draw the boundaries or something.