The order of St Michael and St George is explained in an episode of "Yes Minister"
Hacker: But hasn't Humphrey got enough decorations already?
Bernard: Well, naturally, he's got his G
Hacker: What's his G?
Bernard: You get your G after your K
Hacker: You speak in riddles, Bernard
Bernard: Oh thank you, Minister
Hacker: That wasn't a compliment!
Bernard: Well, take the Foreign and Commonwealth Office, for example. First you get your CMG - Companion of the Order of St Michael and St George. Then you get your KCMG - Knight Commander of St Michael and St George. Finally you get your GCMG - Knight Grand Cross of the Order of St Michael and St George. In the Service, of course, CMG stands for Call Me God, and KCMG stands for Kindly Call Me God.
Hacker (chuckling): I see. And what does GCMG stand for?
Bernard: God Calls Me God.
But to answer the question - no - a Companion of any order is NOT a knight.
Within the Civil Service there is actually a table of the "appropriate" honour to be awarded to any grade:
Administrative grades: MBE (Member of the Order of the British Empire)
Grades 6 & 7: OBE (Officer of the Order of the British Empire)
SCS1: CBE (Commander of the Order of the British Empire)
SCS2: CB (Companion of the Order of the Bath)
SCS3: knighthoods
Quite appalling, really, when you think about it ... that the level of honour should be determined by the grade at which they are employed