http://en.wikipedia.org/wiki/List_of_countries_and_dependencies_by_population_density

Above is a link to a list of places ranked by population density. THe UK is 52nd on the list.

Even if the UK were over-densely populated, that would be because there were too many people there. It would tell us nothing about which of those people constituted the excess.

Here is a synopsis of a study that proposes that modern cultural behaviour emerges when a threshold population density is reached - and eve tnat it may disappear if density drops below that threshold. Tinker with population densities at your peril!