I kitted out a female friend with my spare pair of pants a few times. Unless you are like 6" tall, Small would be fine length, you will most likely find that the legs are way too baggy and the waist is too large though.
I seem to remember a while ago from a post by an american female baller that Invert are supposed to be a pretty good fit for girls. Try before you buy obvs though. Where abouts in the UK are you? I'm sure if you get yourself to a store they would be happy to helpfind something with the needed padding that doesnt simply fall off you or trip you up.