SELECT MAX("BR No.") FROM table_48880 WHERE "SECR No." = '765'
What is the highest BR number with a SECR number of 765?
CREATE TABLE table_48880 (
    "SECR No." real,
    "SR No." real,
    "BR No." real,
    "Builder" text,
    "Date Delivered" text,
    "Date Withdrawn" text
)