I am size 14, 47 years old and I have a dress similar cut to the phase 8 dress and it is really flattering on my with the long balloon sleeves, very neck and the flare below the fitted waist. I get compliments when I wear that dress, and I think it suits a wide range of ages, but is particularly flattering on a 'mature figure' ie, a woman with breasts and hips
I saw another stunning dress recommended on here very similar cut in green and other different colours, but for the life of me can't remember the brand.
Some of the other recommendations here I think could classify as 'granny chic' and would look great on a younger person, like the name Ethel is super cute on a baby, but just old on an old person. You would have to be uber fashionable with wild accessories to pull off granny chic, when you are actually the age of a granny.
If you have $200, sorry I know it's pounds,, my phone and I am abroad, and you never will wear the dress again, what about hiring an $800 dress? I imagine if you go to a hiring agency, the staff may be honest about what really suits you because they have a range of items on offer, and their motivation is a happy potentially repeating customer rather than selling you 1 item from their own store to hit a sales target. More eco concious too.