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Hello do you wear these over your underwear or can they be worn on their own under a dress?

I'm a 28 inch waist which is uk side 10, which size should I order? Size guide says M is a uk 10 but 3XL is a 28 inch waist so it doesn't make sense?

Which size should I order as a uk size 14 ?

What size would you recommend for a 16/18?