Taken from here
To qualify for naturalisation as a British national you need to have at least one year with permanent residence, i.e. 5 years to get PR plus 1 >with PR = 6 to qualify.
Please see this link:
One of the requirements is that you:
· “had indefinite leave to stay in the UK for the last 12 months (or permanent residence if you’re an EEA national)”
If you want to be more precise go can see the Schedule 1 (2) (c ) of the BNA1981 which says:
(c)that he was not at any time in the period of twelve months so ending subject under the immigration laws to any restriction on the period for >which he might remain in the United Kingdom;
Sometimes the info is very confusing but you need 12 months with PR to qualify.
You need to apply for Permanent Residentship (5 years required) first and then after a year for Citizenship.
In regards to proving your 5 years:
One form that is accepted is letters from employers, educational establishments or other Government Departments indicating your presence in the United Kingdom during the last five years.
If you were employed, letters from your employer. If not, letters from Department of Work and Pensions stating you were receiving benefits.
You also should have registered (1k fine if you didn't) on the electoral roll in your council.
(This one I am not 100% sure about, but I have heard people talk that it's also acceptable, which makes it easier than finding all P45s and going through all employers that might gone bust in those 5 years...)
More info in this guide