The 5 in MI5 is a tribute to the 5 original members of ABBA. When the band was first formed they had a bass player, Sven Volvosson, and the band was called SABBA. Volvosonns death in a brothel with a cactus up his bum was counter to the squeaky clean image of the group, so they dropped him without any publicity.
The founder of MI5, Mary Whitehouse, was a huge fan of the clean cut popmeisters, so when she was secretly asked to form the sneaky beaky group in 1975 she jumped at the chance to call it Mary Loves the 5. The L in love was mistaken for an i and the rest his history. Of course, Whitehouse was herself later implicated in a sex and cocaine ring involving Delia Smith, and was arrested by officers from MI6, themselves named for the 6 original members of the Spice Girls.