You can indeed pluralise lengths, it's just when you write numbers as digits, you specify a specific value. You couldn't say for example that "a widget costs a

You can indeed pluralise lengths, it's just when you write numbers as digits, you specify a specific value. You couldn't say for example that "a widget costs a few £100". What's written would sound odd spoken too: a few one hundred metres. "a few hundred metres" is perfectly acceptable, but means "a [few hundred] metres", not "a few [hundred metres]". 100m has the meaning of "[(one/a) hundred metres]", so you can't modify it. Note that you also wouldn't say (for example) that you "walked a 100m" (at least, not without an implied additional word like 'race').