What's the allowed format for UK National Insurance Numbers?

The criteria for a valid National Insurance Number are the following:

• It must be 9 characters long and start with two letters, followed by six numbers and one letter e.g. AB123456C.
• The first and second characters cannot be the letters D, F, I, Q, U, and V.
• The second character cannot be the letter O.
• The first two characters cannot be any of the following letter combinations: BG, GB, KN, NK, NT, TN and ZZ.
• The final character must be either A, B, C or D.