As Katheryn was taken from Syon Abbey to the Tower of London by river, yes, she would've passed under London Bridge.Whether she actually looked out of the window of her barge to catch sight of the mounted heads, we don't know. No contemporary sources mention that she did (or didn't).
Roland is correct about the route and the fact that Catherine would have had to look out purposefully to see them. Maybe she did; hopefully she didn't. It's also worth noting that by the time the barge reached the Bridge from Syon Abbey, it would have been well into the late afternoon, if not the evening, and it was February - so it's highly likely that by the time she reached their heads, it would have been too dark for her to see them anyway.
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