Hiya
Lots of inetresting and useful tips, ladies.
I have been riding to work for a few years now (includes the UK and New Zealand). I tend to average around 20 to 25 km each way at the moment ( I have two hospitals I am based at) and have to get around the "I don't have a permanent locker" too.

I use a panier to keep some weight off the shoulders, but on my racer I would probably have a seat-post based bag (they can take up to 8-10kg and can be bought online if your LBS does not stock them).
I manage to take clean top, trousers, shoes etc on a daily basis in a small, light back-pack. I use sample sized shower gel bottles (or ones from hotel stays that can be refilled) travel deopderants and mini face cream pots etc. It is relatively easy to do and look smart (I am a doctor and need to look the part in clinics) I feel great every day because I arrive all endorphined and pious!

The other option of, taking it easy on the way to work is valid if the distance is short, but nothing beats that fresh just-showered, just- exercised feeling.

Keep trying alternatives and settle on what works for you.