The Home Office kept duplicates of the certificates of British nationality and they are available in the National Archives series HO334, recording the:

These certificates are generally known as 'R' certificates and refer to registrations of British citizenship declared by a British subject of a colony, a protectorate or a protected state.

The catalogue to the HO334 series is arranged by certificate number rather than by name. The name indexes to the certificates are still held by the:
Home Office Immigration and Nationality Department
Liverpool Nationality Office
B4 division
India Buildings
Water Street
Liverpool L2 0QN.

Restrictions apply to those issued less than 30 years ago.

The National Archives record HO334/1988-2445 covering the period 1973-81 is for certificate numbers 300001-414400 (these seem to be purely Pakistanis).

HO334/2446-3436 covering period 1973-82 and 500001-857706 are for other colonies/Commonwealth countries with a few Pakistanis, foreign women married to British subjects and minors. These files are still closed under the 30-year closure period.