So the UK isn't always the final destination either.
you are absolutely spot on
however Trevek was asking whether in theory they ought to be able to go to the UK
and the answer appears to be a resounding yes
if you are an illegal, speak Polish and are settled, what point is there to go to the UK
if you are a student who overstayed their visa, hustling just to survive (excuse the 1970s US ghetto parlance)
and speaking English, you might be better off in Blighty