United Kingdom - Passport & Nationality - British Citizenship by Registration - 1981 (4B)
Holders of certain forms of British Passports, who hold NO OTHER nationality or passport of another country, may apply for full British Citizenship under this provision.
Please note: do not assume that if you do indeed hold nationality of another country that this will necessarily exclude you from upgrading to full British Nationality. It may be that your existing citizenship can be voided or invalidated (in other words, it may have been granted or applied for in error), thus allowing the Registration as a British Citizen.