No one can be illegal – only actions can be made illegal. There is no such thing as an ‘illegal’ asylum seeker. Seeking asylum is not a crime.
Most asylum seekers do not ‘fail’ to get asylum – they are refused asylum by the Home Office. The fact is that the Home Office has the policy to refuse asylum to as many people as possible. Similarly people who have been refused asylum are sometimes removed to another country by the Home Office. They are not ‘deported’. Only people who have been convicted of a crime are deported.
People who have been refused asylum have not broken any law or committed any crime. Being refused asylum does not make anyone a criminal or ‘illegal’. Only people who have been found guilty in a court of law can be called criminal.