Just wanted to give another recommendation for ASOS - good range, good prices, lots of sales, free shipping unless you want it faster than a week. I have some friends who use Marks & Spencers, and I absolutely love Uniqlo - it's the closest thing to Old Navy as far as I'm concerned for things like office clothes, basics, and jeans.
Not sure if you're interested in shoes but there's Javari (Amazon spin-off).