Bridal Room Horsell in Woking stocks Justin Alexander, and so does a shop in Bromley (but I didn't have the best experience trying on dresses there..) if you're in the east of surrey it's not too far, or up in London at London Bride.
It sounds like you've tried on a full mermaid dress and like that shape, though, but if you're still looking for the dropped waist, not-quite-mermaid, it looks like JA and Enzoani have the most options.
I'm personally not a fan of the mermaid style, but I do think it looks good on some people, it's just a personal opinion, and based on my size 16 self (18 in a wedding dress!). It's really difficult to know what these dresses will look like on a real person, though, as these models are damn near anorexic, and most brides won't have hips bones poking through the dress!

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