"Frank Green UK" is a company that sells reusable cups and bottles. However, reviews from customers are mixed. Some customers have reported issues with the products, such as the lid retaining water after washing, the cup retaining and worsening odours, and the cup not being leak-proof. There are also complaints about customer service, with some customers reporting difficulties in getting a refund for faulty products. On the other hand, some customers have had positive experiences with the products. Therefore, it's recommended to read multiple reviews and consider these factors before making a purchase.