United Kingdom Size 6 to US: 18-20

United Kingdom Size 6: Waist range of 60–64 $\text{cm}$ (23.6–25.2 $\text{in}$), hip range of 86–90 $\text{cm}$ (33.9–35.4 $\text{in}$), lengths (Mini/Midi/Maxi): 42/75/100 $\text{cm}$ (16.5/29.5/39.4 $\text{in}$), and fit style Fitted Waist / A-Line & Straight.

US: 18-20: Waist range of 91–99 $\text{cm}$ (35.8–39 $\text{in}$), hip range of 117–125 $\text{cm}$ (46.1–49.2 $\text{in}$), lengths (Mini/Midi/Maxi): 51/84/109 $\text{cm}$ (20.1/33.1/42.9 $\text{in}$), and fit style Fitted Waist / A-Line & Straight.