You're never too old for TopShop: if you can fit the clothes, you can shop there. Likewise, I think, for New Look and River Island.
Seriously, it's one thing to decide you want to change your style because you feel you've changed and it no longer expresses you, it's another to start getting angsty about being "too old" for things you like. There is huge cultural pressure on women (and you can double that for mothers,) to conform in some way to weird, not-so-clear cultural pressures. We tend to express that mostly around "age".
Think hard about what you mean when you say "too old". Deconstruct it a bit. Ten to one, sexuality, and looking sexy, might be at the bottom of it. Or something even weirder about looking "respectable", "dependable", "like a mum". Or even - and this is a bit controversial - "more expensive": as though you are some kind of walking advert for career success tracking your years (I reckon this can be traced back to ideas about women being living signifiers of a husband's [economic] power). What is that all about?
With regard to your question, I agree with previous posters suggesting you think about your style first, and go shopping second - you'll find the items you want in a range of shops. I like Hush (but I'm older than you!) and tend to look at the on-line catalogue and keep an eye out for cheaper versions of the same items.
By the way, Marks and Spencer have some really nice dresses in the Ltd Collection at the moment, and their trousers can be utterly great. And cheap. So don't dismiss them out of hand. 