I know someone who entered and lived in this country illegally for five years. He decided recently to return to his own country. He simply bought a plane ticket. He was detained for around 30 minutes when leaving the UK to find out what he had been doing in this country for the past 5 years. He was again detained when he entered his own country for around 2 hours, again to find out what he had been doing.

He is now in the process of applying for a visa to re-enter the UK legally, so I assume that it does not affect your future immigration applications.

 

The UK Authorities decides each and every application based on its own merit. And in most cases your past immigration history would not really matter.

A few situation where the history would influence a decision is where you have been previously deported and barred from returning for a period of 5 yrs or a situation where you have a record and seen as a threat to public peace, security and the general public good.