A and C
Stansted has two security zones, the Controlled Zone and the Restricted Zone.
By walking across the grass you were basically walking from the Restricted Zone to the Controlled Zone. There are no security checks as such to enter the Controlled Zone (the home of business aviation), but they are of course required (as you found out) to enter the Restricted Zone. The restricted zone roughly runs from the Alpha's to the Delta's, and from the terminal to the Juliet taxyway (it also includes the Zulu's nowdays).
By walking on the grass by Alpha Six, although you may not have reached Juliet, the security guy probably didn't watch you all the way so had no way of knowing that.