Yes Yooxygen is amazing. ASOS also have the Green Rooms although I am not completely sure that everything featured in there is completely ethical/ sustainable so it is well worth checking out the different brands individually. Zalando also have a good seelction of Eco brands but they don't have an actual eco section so you kind of need to know what you are looking for to start with. If you want an easier way of finding eco clothing
style-is.co.uk has clothes from lots (probably at least 100) eco brands.