I have just been online shopping for this exact purpose this afternoon.
I have found some wrap or button dresses/tunics from Boden, White Company and Fat Face which I can wear with leggings or opaques and boots, and then a few henley tops to wear with jeans - I also do the layer thing, one up one down, or a stretchy vest with a cardi works pretty well too.
Wrap dresses and wrap tops work really very well, though you might not be comfortable with quite the amount of boobage they reveal. If you really want to wear dresses/tunics, wrap or button-down dresses/tunics are the only things that work, really (try White Stuff, though I know they have a reputation for being mumsy...).
Better still are trousers worn with a top over a stretchy vest -- as Gracie says, top up, vest down, voila! Or stretchy cami-vests under neat cardigans. I have a series of stretchy vests in different colours from M&S that work brilliantly; they come in two packs.