"9. Difficulties of removal.
Quite apart from the extensive legal and practical difficulties of removing people against their will, the Government has almost no capacity to remove people who are neither foreign prisoners at the end of their sentence nor failed asylum seekers. The Government claim to be removing an immigration offender every 8 minutes is deeply misleading. This number includes those turned away at the border. The number actually being removed after entry is about 1,000 a month or 12,000 a year (excluding foreign national prisoners). Meanwhile, the number of visas issued has risen rapidly in recent years to over 2 million a year. The Government's removal capacity is less than 1% of this number. Therefore, unless over 99% of those granted visas leave when they are supposed to, despite the absence of any checks, the number of illegals in Britain will climb every year."
From...
http://www.migrationwatchuk.org/briefingPaper/document/162