Consider the law which was in place pre-July 2012 (granting Indefinite stay after 14 years unlawful stay) and the current law (granting 2.5 years visa after 20 years unlawful stay) and find a middle ground – to grant 2.5 years visa after 15 years unlawful stay.
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People live in the United Kingdom unlawfully and they must for 20 years before they qualify for leave to remain (2.5 years visa NOT Indefinite stay).
Those granted with 2.5 visa do NOT qualify for access to public funds, therefore, they are not able to claim benefits.
People living unlawfully in the UK work illegally in exploitative situations. It is therefore unreasonably harsh to perpetuate a degrading lifestyle for 20 years when it could be reduced to 15 years.