Zara FTW when it comes to essentials - plain t's cardigans etc. The Abercrombie and Fitch store store just off Regent Street is pretty good for the odd item.
If you wear demin, can't go wrong IMO with Nudie jeans.

My fav store in general though is All Saints in Camden, plus there are lot of independent stores around it which hold hidden treasures.

If you have cash to burn Aquascutum has some seriously nice gear in there.

I wouldn't say i'm in to fashion though, i just like to make an effort when i go out.