I could maybe do strapless (although would probably wear a shrug type thing or wrap) as those Gossard ultimate strapless bras work well on me. I am happy to flash a bit of clevage, but still want to look classy. £350 would be about my maximum budget, preferably under £200.
This one from Phase Eight should look good with your cleavage. There are some other stunning looking maxi dresses there that might appeal to you.
I'm the same height and size as you and just slightly smaller of bosom and found the Phase Eight dresses tend to be very figure flattering. Although I like the look of the dress you linked, I really don't think I would ever chose to wear it, as with the different coloured top part over ample breasts, I don't think the waist would sit where it's supposed to!