You do not have the height/proportions of a fashion model and so high fashion agencies are not likely to take you on. Look for more commercial agencies as you have a pretty face and lovely smile which are good for advertising. However, do not think that the only way into modelling is through an agency - it is perfectly possible to make a great career for yourself working freelance on sites such as this one.
I did apply to Sandra Reynolds but they wanted £100 for me to do a test shoot with them, and unfortunately I cannot afford that at the moment. So I'm looking for a photography to do a TF shoot, so I can apply again if they photos are good enough.