From the form:-
Note 6 If your partner is a British citizen without a passport, you must provide his/her full birth certificate (showing the parents’
names), accompanied by at least one other formal document as evidence of ordinary residence in the UK for the last three
years. Anyone born in the UK on or after 1 January 1983 will only be a British citizen by birth if a parent is a British citizen or
settled in the UK at the time of birth. In that case, we will need evidence of the nationality or immigration status of your partner’s
parents at the time of birth for us to be satisfied that he or she is a British citizen.
If your partner is a non-British citizen without a passport, you must provide a Home Office letter or other document showing
that he/she has been granted indefinite leave to enter or remain in the UK, accompanied by at least one other formal document
as evidence of ordinary residence in the UK for the last three years.
The following are examples of formal documents that may be provided as evidence of ordinary residence for the last three
years: notice of income tax coding, driving licence, building society savings book(s) /bank statements, National Insurance or
National Health Service registration issued by the Department for Work and Pensions or a local health authority.