Start with the fabric, because everything follows from it
The fabric is the clearest signal of whether a luxury swimwear brand is worth its price. The best swimwear is made from high-quality technical fabric, produced by specialist mills that have spent decades refining stretch recovery, chlorine resistance, and colour retention. These fabrics cost significantly more than the nylon and polyester blends used in mass-market swimwear, and they perform measurably better over time.
A brand confident in its materials will tell you the fabric, or that it is recycled to a specific standard, because those details are part of what justifies the price. Vague language about premium quality without specifics usually means there is nothing specific worth naming.
Some of the strongest UK luxury swimwear uses recycled technical fabric that meets the same performance standard as virgin material. This is worth seeking out: it performs identically and meets a more demanding environmental standard at the same time.