The final two digits of the Driver Number are officially classed as the "licence numbers"
http://www.dvla.gov.uk/media/pdf/leaflets/inf4 5.pdf
If they are used as a gender indicator then this seems a bit redundant since females are already identified by the first digit of their birth-month being increased by 5;
e.g. using the Driver Number format
SSSSS
ymmddyFF
cCC
xx
S = surname
F = forename(s)
y = year of birth
m = month " "
d = day " "
c = check digit
C = check letter
x = 'licence numbers'
John A Smith born 1st Feb 1934 would start
SMITH3
02014JA... etc.
Janet A Smith born 1st Feb 1934 would start
SMITH3
52014JA... etc.
I would have thought that the 'licence numbers' would be put to better use as some kind of 'issue number' whenever the photocard is updated/replaced such as the 10-yearly photograph renewal requirement.