[She-Ra]

I think the main competing factor of over population is breeding - not or not only movement of people. Britain as unified country didn't open its borders, it realised that it had made an empire and had the ability to spread its core base to wherever it wanted to and then had to allow its other subjects to move within its empire as well. That then spread to refugees/asylum seekers and rich immigrants from other areas where Britain didn't have overt control over but usually had a financial influence with (there are plenty of areas that don't get such opportunity but Britain doesn't have a hand in the pocket so why bother?). British people are themselves a mix of races with very few 'pure' indigineous people left and those few and their later mixed children were travellers - and it has to be acknowledged that just because you go to a country doesn't mean you can lock the door behind you. Immigrants from the last 'settled'/accepted group of British people considered to be British e.g. Victorian thereabouts (and not including those who became or were born citizens afterwards), are still a tiny percentage of the overall population but their numbers with the accepted British people have over bred and sexuality glamourised without responsibility and practicality of what having children really entails to the individuals involved, society and the surrounding enviroment. Over breeding is an age old problem preceding governments 'allowing' immigration and has caused the same scenario in most reasonably habitable places worldwide.

Note - it's interesting that people more readily accept conqeured and hence the mixed breeding that follows as an accepted race of a country moreso than people who come after.