VP was issued from July 1928 till May 1929 (10 months to issue one under 10,000 registrations), then OF between May 1929 and April 1930, then OG between April 1930 and April 1931, then OV between April 1931 and May 1932, then OJ between May 1932 and May 1933. The registration system in Birmingham was running out of availabilty, so previously unissued OC was then allocated to Birmingham Licencing Office and ran from May 1933 till March 1934, by when there was becoming a national shortage of registration series. It was decided to start a three letter sequence, effectively giving an extra 23 x 999 numbers per series (I, Q and Z prefixes were not used in mainland England). So AOA ran from March 1934, BOA from April 1934 etc. Multiple series were issued at the same time, sometimes restricted to certain vehicle types like Commercial vehicles, or Motorcycles, but with only 999 individual issues to a letter series they were gone through quite quickly. Eventually this system ran out too, and so "Reversed" registrations were used - 1 to 4 numerals to 1 or 2 letters, then 3 numerals to 3 letters. The rate of vehicle registration was multiplying so fast thet this could hardly keep up, so a new system started using year letters, initially behind the unique registration and later in front of it.
Now we have a system using two letters as an area of issue code, two numbers to define a half-yearly date, and A three-letter sequence which uniquely distinguishes each of the vehicles displaying the same initial four-character area and age sequence. The letters I and Q are excluded from the three-letter sequence, as are combinations that may appear offensive (including those in foreign languages). Due to batch allocation of new registration marks to dealers, it is common for cars with "neighbouring" letter sequences to be of the same manufacturer.