There are also certain categories of British passport which require a visa. GBR is normal British passport issued to a British citizen. Other categories, as below, may not qualify under ESTA.
GBR BRITISH CITIZEN
GBD BRITISH DEPENDENT TERRITORIES CITIZEN
GBN BRITISH NATIONAL OVERSEAS)
GBO BRITISH OVERSEAS CITIZEN
GBP BRITISH PROTECTED PERSON
GBS BRITISH SUBJECT
PCN PITCAIRN CITIZEN
SGS SOUTH GEORGIA AND THE SOUTH SANDWICH ISLANDS CITIZEN