United Kingdom - Permanent Residency - 20 Year Long Residency Concession
The main requirement to be met by an applicant is on the grounds of long residence in the United Kingdom is that you have had at least 20 years continuous residence in the United Kingdom, excluding any period spent in the United Kingdom following service of notice of liability to removal or notice of a decision to remove.
There is no requirement to have held a valid UK visa. In other words, time spent in the UK may include illegal residency.
This will allow the applicant to obtain a 2.5 year residency visa, which they can renew three times (i.e. hold for a total of ten years). At the end of the ten years, they can then apply for Indefinite Leave to Remain.