Piper,why don't you ask one of the underwear advisors at M&S to help you find something to suit what you're looking for? My DD was having a terrible time trying to get a good bra to support her 'girls' (she is around a D cup but has a tiny back) and I eventually persuaded her to come with me to marks. The lady measured her,came back with 2 choices and both fitted her really well. She had virtually given up finding anything comfortable and was so pleased. I don't know if they have these trained ladies in every branch but surely in all the larger ones?
I have a very comfortable spanx I bought to keep everything smooth under my fitted wedding dress. It's lasted me 5 years so far as it doesn't get worn very often. I wore it under a close fitting dress last weekend and it gave me confidence-made me stand up straight,ironed out any bumps and made sure I had no glaring undie lines. I am not really fat,but certainly could do with toning up,and I find the spanx make a real difference but are pretty comfy too ( I have the high wasted shorts. Take some getting on but I have the knack now!)