Oh, it's because I think Tulip, as a flower, is a rather undignified name for the head of MI6. So it embarrassed her and that's why she didn't use it, and Alex agrees it would be embarrassing so he understands.
(Not that Tulip is a bad name or anything. I mean, it's a little bit awkward, but it's all right if you're not head of MI6. Generally, most flower names would be awkward for someone leading a secret services agency. Tulip, Daisy, Lobster Claw...)
Ah, I rambled on. Anyway, welcome to the board, SpottyPup