Sidney is still very much a male name in the UK. Whilst there might be one or two female Sidneys, it is overwhelmingly seen as an older mans name. Ashley is still a boys name here too. There are more female Ashleys than Sidneys but it is still seen as a perfectly acceptable boys name. I know several male Ashleys but no female ones.
I'll always think of Ash as a boys name, but I think I'm more likely to think of Ashley as a girls name.
Ash -> Ash Ketchum from Pokemon, Ashley -> Ashley Simpson (who I actually have no idea of who she is/what she does).
Ash Ketchum who was named Sacha in France, a name I don't really like and even though I know it's a perfectly valid boys name for me it's scream 'girl' because it ends with an 'A'.
Also, how on earth is Sacha and Sandy short for Alexander?