The Verdict: Need More Info
Status lost by operation of law can often be put right, most often under section 4L, which has no deadline. Establish the exact legal event that ended the status, and its date.
The usual events: a colony's independence, which removed citizenship of the UK and Colonies from many with a connection to the new country; marriage to a foreign national before 1949; or a parent's change of nationality before 1983. Officials also gave wrong advice for decades.
Section 4L lets the Home Secretary register an adult who would have been British but for historical unfairness, a public authority's error, or exceptional circumstances. Section 4B is an entitlement if you now hold no other citizenship, and section 4C covers losses through a mother for births before 1983.
Check whether you lost it at all; transition rules were often misapplied. Bring dates and documents to an adviser rather than a family account.