Outlet stores, but at weird times and in the chavviest places (no offence, I'm serious). So - don't go in the January sales, cos that's when everyone else is bargain hunting. The middle of February, however, can result in some serious, shops just want to get rid at all costs, bargains. Similarly, Bicester Village is not that great any more, even at less expensive shops, cos it's just constantly busy with rich Chinese tourists and doesn't have to try very hard. In contrast, somewhere like The Galleria in Hatfield, or the Freeport one in Braintree are not teeming with rich Chinese tourists and thus have to try harder.
I really recommend Jaeger (outlet stores only of course). The quality is fantastic, the cut forgiving for non size 10 people, and if you time it right you can get some serious deals. I have numerous dresses from Jaeger that cost £250 new, but that I bought for £50. Similar price reductions on skirts and tops too. My best ever purchase from Jaeger was a navy pencil skirt, completely classic, reduced from £99 to £13.
I know you say you can't afford Hobbs, but it is great quality and in the sale or in outlet stores you can get fantastic deals. I have spent quite a lot at Hobbs in the sale, but now have lovely things that were greatly reduced (eg, silk top reduced from £99 to £29, a lovely silk dress reduced from £159 to £47), that will see me through this Spring/Summer.
The thing about building a good work wardrobe from sales and outlet stores, which is the only sensible option as far as I can see, is that it does require an investment of time as there will be many times when outlet malls are full of rubbish, and you will also need to be prepared to buy things out if season. But if you can do that, I think in the long run it will be better value and more sustainable than buying cheap and cheerful things from places like H&M.