I've seen a dress that I really like, but I would need to buy it online. I'm normally a 16 to 18 in Monsoon or East and a 14 in Wallis. I measured myself and looked up their size charts, I'm between 12 and 14 on their hips, 16 on their bust and off the scale on their waist (apple-shaped). Should I go for an 18 or are they quite generous in their sizing? It's this dress, so not very fitted on the waist.