I never used anything like this for FLR just a mortgage letter confirming they own the property
11. MAA11 Adequacy of accommodation
The ECO’s judgement should be based on the evidence from the applicant. If the ECO is not sure of the credibility of the applicant, he / she should ask to see a letter from the owner of the property (which may be a housing authority, housing association, landlord or a building society). This should confirm particulars of tenure and occupation of the dwelling, together with a description of the accommodation and, if rented, a copy of the lease.
The onus is on the applicant to provide confirmation that there is no objection to an additional resident moving into the accommodation.
A letter can be used from the Blding Society to confirm tenure. Just be aware many Mortgage Agreements do have Clauses which require the Lender's Consent if other Adults over 18 are to reside at the Address.