Yellow, long evening dresses in London??

[giuliaplus2]

Hi there,
so as I've said in another post before I am going to a wedding this March. I've got my dress already, but my sister is struggling to find hers! She's going to be a bridesmaid and supposed to be wearing a yellow and long dress, since it's going to be a beach wedding...
Can anyone recommend a store here in London that might have one? She said it shouldn't be too expensive.. Her absolute maximum is ￡150, but she's hoping to find something below ￡100. Can anyone recommend a store where you know they do have yellow dresses? She has been to quite a few already, but it's really not the season for yellow dresses

Any help would be appreciated!