maybe there are several who manage to dodge the system and in truth most come form china the usa and britain
however of the many (50000 odd) who do come here by plane all have a visa and all have passports.
the other factor is that there was none who came from the countries such as iran or afgahnistan (based on 2011 figures)
on top of that most would actually work to sustain themselves
"Three in four of the 58,400 visa overstayers came on tourist or holiday-working visas; one in seven arrived as students and one in 15 disappeared after being granted temporary residency.
Last year, only 6720 visa overstayers were sent home, most voluntarily, after applications to stay longer were rejected."
