I’ve never ordered their bikinis @xxcsb, only gone for their clubwear dresses. The dresses come up a quite small. A size XS-M I’d say is good for up to a size 10, I order an L-XL (I am a 10/12) which fits cos they’re stretchy.
Here’s their size chart
Following size chart is for reference only. Please allow 2%-4% error.
Small: UK6, EUR 32, Height (150-160cm/59-62″), Weight (40-45kg), Bust (29-31″), Waist (22-25″), Hips (29-32″), Cups (75A/75B/70C)
Medium: UK8, EUR 34, Height (155-165cm/61-64″), Weight (45-50kg), Bust (31-33″), Waist (25-27″), Hips (33-36″), Cups (80A/80B/75C)
Large: UK10, EUR 36, Height (160-170cm/62-66″), Weight (50-60kg), Bust (33-36″), Waist (27-30″), Hips (37-40″), Cups (85A/85B/80C)
X-Large: UK12, EUR 38, Height (165-175cm/64-68″), Weight (55-65kg), Bust (36-39″), Waist (30-33″), Hips (40-43″), Cups (85B/80C/85C)