i don't think american equivalents of high-street fashion ever really translate well in the uk, to be honest. they could be okay, but never as successful as continental exports such as zara, mango and h&m, amongst others. it could just be my experiences but people tend to "binge" in the same american shops and end up looking massively douchey (urban outfitters, abercrombie & fitch, hollister, i'm looking at all of you). in the eyes of american brands, people are compartmentalised and social groups probably resemble that of glee. urban outfitters - maybe a girl who likes quaint jewellery isn't a hardcore indie kid who doesn't want to team it up with curtain-esque florals all the time, and deserves a little more choice?
i know that to some extent, every shop has its own target market and you would probably have to fit within that market in order to like what they have. but, within british/european shops, i feel like the target audience is much more wider and encompassing. american brands seem to box up people into very specific categories - hipster, 14 year old teenage girl, sporty and so forth - and if you don't fit into that, you'll be hideously hard-pressed to actually love anything. british brands do appeal to certain styles (they have to, for marketing!) and as much as topshop pisses me off at times, it is an example of different types of people all coming within one shop and actually finding something that's for them - and it is that which i find to be lacking in american exports.
forever 21 is not this type of place.
my advice? go in, ignore any piece of fabric you see, and go straight to the second floor. it's the only part that is worth your time.