What is a UK citizenship certificate?
That would be the certificate of citizenship issued when a foreign-born person naturalises as a UK citizen after living in the UK for a number of years on qualifying visas.
It won't apply to your fiance as he is a UK-born citizen.
Did you need letters from friends and family stating that your relationship is genuine?
No, you don't. They cannot be considered and will not be looked at.
The only letter you need is the mandatory 'Sponsor letter of support' which needs to be written by your fiance (your UK sponsor) and outline how he meets each requirement of the visa in order to sponsor you.
The letter should contain:
Paragraph 1: how he qualifies to be a sponsor (his UK citizenship)
Paragraph 2: how he meets the financial requirement... or in his case, why he is exempt from meeting it and how he will be showing Adequate Maintenance
Paragraph 3: where you will be living together in the UK... full address and property details, and confirmation that you have permission for the visa applicant to live there too
Paragraph 4: a short, factual history of your relationship and why you are choosing to settle in the UK together
