There is an agreement between the European Union and Turkey that gives Turkish nationals special rights. In the past the EU had association agreements with countries such as Bulgaria, Romania, Poland and other states that have now joined the EU. These allowed nationals of those countries to work in self-employment in the UK. Nationals of those countries, including people who came to the UK under the old association agreements, now have the right to work as self-employed people in the UK because they are members of the European Union.
The Ankara Agreement (an Association Between the Republic of Turkey and the European Economic Community) is an agreement that aims to pave the way towards the accession of Turkey into the European Economic Community. The Association Agreement with Turkey provides rights for Turkish nationals and requires the Home Office to apply old, and more generous, rules to them. The UK Home Office's original practice in interpreting the agreement as applying only to those who are already in the UK was changed in 2009 to open a route for Turkish nationals to apply for visas to come to the UK under the terms of agreement. The Ankara Agreement is not extended to Turkish citizens that have entered the UK illegally.