The main idea of this poem is spelled out at the end. Throughout the initial stanzas, Motion uses imagery of entrapment and imprisonment. The streets are guarded and the rooms are "sunlit" implying that the speaker understands that Anne Frank wouldn't want to be in a sunlit room, she'd want to be out in the sun. These are all things that we take for granted.
In the last stanza of the poem, Motion illustrates the guilt feelings of the speaker. Anne cannot leave,
as simply as I do, and walk at ease
up dusty, tree-lined avenues, or watch
or watch a silent barge come clear of bridges
settling their reflections in the blue canal.
The speaker illustrates that Anne Frank's simple wish, of leaving whenever she wants to, is something she was never able to do once she went into hiding. Motion is illustrating the main theme of imprisonment versus freedom, "one enduring wish ... / to leave as ... I do, and walk at ease." This is what we take for granted and what Anne Frank didn't have.