If you are searching the civil registration indexes on freebmd, you can click on the district name and the District Info page will open telling you which county/ies that district is in. If you then click on the blue word 'here' at the end of the line, you will get a list of all the parishes in that district.
The volume numbers are also a clue: 9d is
Yorkshire, 5a covers Dorset and
Wiltshire, for instance. The volume numbers start in
London with 1a being central
London, 1d is south of the river
Thames, then the numbers radiate out to cover southern
England. The volume numbers then get higher as you move north, up to 10 at the Scottish border. Wales is last, being covered by volumes 11a and 11b.