What is the main theme of the poem "Anne Frank Huis" by Andrew Motion?

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.