For bras you can try out Amaya Lingerie at Holland Village - Holland V Shopping Mall, 2nd floor - they are selling European brands and cutting for bigger cups is very comfortable. I bought one set (I'm size 90D) to try out and now it's my preffered set

I have bought one set from M&S too, but there most of designs are granny style for big sizes and in fact my size I have seen there very seldom.
But for clothing Dorothy Perkins time to time has good selection.. It depends how old are you and what kind of style you prefer. At Robinsons there are big sizes but designs are not very appealing to me and expensive though.