Leslie is mostly a male name in the UK. I've met quite a few, all called
Les, and all over 60 - the only female Leslies I've seen have been actresses
Leslie Ash and
Leslie Caron.
Lesley used to be quite common on girls here, but I've never seen one under 40, so yes, it does seem pretty dated. I don't think it's awful, but I'm not a fan of it.