For the most part, "London" is written in iambic tetrameter. This little meter is very similar to iambic pentameter, except that, instead of five iambs there are only four iambs (tetra means four, so tetrameter means four of the same meter). Now, an iamb is a beat that consists of an unstressed syllable, followed by a stressed syllable. It sounds like daDUM (if you say "allow" out loud, you'll hear an iamb). For example:
Near where the charter'd Thames does flow. (2)
Voila, perfecto, neato-frito. Ladies and gentlemen, iambic tetrameter.
Sadly, not every single line in this poem is so perfect. Take a glance at line 11:
And the hapless Soldier's sigh.
You'll notice that the first beat (or foot) contains two unstressed syllables followed by a stressed syllable, while the last two beats are your regular, run-of-the-mill iambs. Come to think of, there are a number of other lines that contain only seven syllables (like this one), including lines 4, 9-12, and 14-15. Many of them also contain anapests (two unstressed syllables, followed by a stressed syllable).
Okay, so what are we to make of this metrical variation, of which our friend William Blake was quite fond? Well, we have a few different answers for you. The shift from 8 beats to 7 is quite noticeable. In fact, it's interesting that the first time this occurs is in line 4, precisely when Blake is talking about "marks of weakness" and "marks of woe." At that exact moment, the meter itself becomes weak—7 is less than 8, right? The same is true of the poem's third stanza, in which every line contains only 7 syllables. That stanza might be read as a further description of those same "marks."
Our astonishment at this little metrical hiccup mimics the speaker's own astonishment at all the weakness and woe around him. Alternatively, you could say that the line itself is plagued by a weakness—it has one less syllable than it should. It is sick, incomplete, imperfect—just like all the people the speaker meets.
Here's another little factoid. There are exactly 7 lines that contain 7 syllables. Well cheers for that one William Blake. In a poem of 16 lines, this is almost half of the poem. This division between types of lines reflects the divisions and fractures that the speaker sees everywhere in London.
It also reminds us of one of Blake's other major themes: the ways in which social life is constricted, confined, repressed, etc. etc. Think of words like "charter'd," "manacles," and "ban," for example. In other words, just about half of the lines in this poem are marked, to use one of Blake's own words, as restricted or compressed or confined. 7 is less than 8, which makes those lines with fewer syllables seem, well, a little restricted.
If it sounds like we're repeating ourselves, that's because we've been infected by the repetitive spirit of this poem. Notice how many words appear two or more times ("charter'd," "marks," "Infant," "cry," "street"). Notice also how the poem rhymes—this too is a form of repetition. In each stanza, every other line rhymes (which gives us a rhyme scheme of ABAB CDCD EFEF GDGD). The repetition of sound suggests that what the speaker sees around him is cyclical or repetitive—that the evils of London will continue to persist. This is nowhere more evident than in the fact that the lone E rhyming sound of lines 6, 8, 14, and 16 ("fear," "hear," "hear," "tear") occurs in both the second and fourth stanzas.
So, in more ways than one, this poem's form and meter either advance or reflect the ideas it's trying to get across. Well that's pretty neat now isn't it?