If you read the chapter about the Lady of Shalott in King Arthur and his Knights of the Round Table, it really helps you understand this poem better.
The simile is a comparison of two unlike things using the word like or as. The trick here is not assuming because these two words are used, a simile is there. In this poem, the word as is used often, however there is only one true simile where as is used.
Here are the similes:
The gemmy bridle glitter'd free,
Like to some branch of stars we see
Hung in the golden Galaxy
In this stanza Tennyson is comparing Lancelot bridle to a constellation in a golden galaxy. Clearly, he is trying to help us see how shiny the bridle must have appeared to the Lady of Shalott as she watched in her magic mirror.
The helmet and the helmet-feather
Burn'd like one burning flame together,
This simile again demonstrates how striking he must have appeared. The next simile appears right after the description of the helmet and helmet feather:
As he rode down to Camelot.
As often thro' the purple night,
Below the starry clusters bright,
Some bearded meteor, burning bright,
Moves over still Shalott.
This comparison compares Lancelot's ride through Camelot to a meteor streaks through a purple sky. Note the word as is used to create this comparison.
And down the river's dim expanse
Like some bold seer in a trance,
Seeing all his own mischance --
With a glassy countenance
Did she look to Camelot.
The Lady of Shalott, realizing that Lancelot loves another, prepares to die knowing that if she leaves Camelot the curse will kill her. Knowing she will never have Lancelot, she does so anyway.