The best document to confirm your right to permanent residence in the country is a Biometric Residence Permit (BRP). It is a plastic card with your biometric data, name and immigration status.
If you are a permanent resident, your BRP will say either:
Indefinite leave to remain – a right to stay in the country without any time limit.
Indefinite leave to enter – a right to enter the country without any time limit.
No time limit – unlimited duration.
Permit residence – a right to live in the country without any time limit (it is usually for EU citizens).
Pay attention to the fact that it is your residence permit that is given to you indefinitely; a BRP card is valid for a maximum of 10 years after which, according to the UK rules, it must be extended. Our advisers recommend that you apply for a replacement BRP three months before the current one expires.