I wouldn't begrudge picking up something from a designer that's a real staple if its a nice style and fit - jeans, blazer, coat, watches (well not designer, but decent watchmaker), but anything with a hint of seasonal, where quality isn't an issue or something that you're not entirely sure on then don't worry. You can get pretty good quality without having to cry when you leave the till. If you really want designer bits then have a rummage in TK maxx or something, some occasional gems in that place! You can always go mid-way, I got some belting jeans from gap that are leagues apart from most high street quality and falls far short of designer prices.
People rave about all saints on here, and as nice as some of the bits are, nobody should charge £40 for a printed t-shirt!