Well, Canada and Australia do, with their points-based immigration systems, so why not Britain?
Unlike Britain Canada and Australia have very large buffers between themselves and illegal immigrants. Central Americans and Mexicans would rather stop in the US especially with Obama's policies of not deporting anyone. Australia has a huge ocean to prevent illegals from just showing up. Britain used to have all of Europe as a buffer but with the EU rules on free movement that buffer is now 20 miles of the English channel and nothing else.