I'm around this weight and you sound like a similar build to me (34DD/E chest, 29" waist, 39" hips).
I'm a comfortable 12 in most shops. (M&S sizes are really big so I'm sometimes a 10 in there.)
For anything with buttons or a tailored cut I'd recommend someone of my build to buy from somewhere like Bravissimo where the clothing is cut to accommodate larger breasts while still fitting normally elsewhere.
Alternatively, I sometimes buy a size up and get tops or dresses altered to fit me. (I find that if you wear tops and dresses in 14/16 just to accomodate a large bust they tend to gape at the armpits and swamp the waist.)