There is a 30% sale on AP in Harrods now, I have stocked up on stockings. Also 50% off Intimissimi which I find perfect for work,as you can buy a 'luxurious' looking set for 20, and look nice and classy without paying lots and also if you accidentally destroy or loose one piece it's not the end of the world.
I find Primark good for cotton bras and knickers to walk around the house and going shopping feeling comfy, and the hold ups are alright. Can't expect too much from 2,50 hold ups. I normally get 4-5 wears out of 1 pair.
Also, TK maxx sometimes has real diamonds, I once bought a myla set for 30, and brands like Elle Mcpherson which I would never pay the full price, but for ?15 I might be tempted.