I take a uk size 11 order that size but they were too small has anyone bought UK size 12?

Crocs™ get looser as you wear them. If they just pinch slightly when new, they will likely be a perfect fit after a couple of weeks. If you really find them tight send them back for a bigger size. I've always found sizing Crocs™ a little sketchy.

Are these really flexible or hard?

I am not sure what you mean by flexible. They are not really flexible compared to, say, surfing shoes. But my daughter is comfortable wearing them and they are ideal for use around the pool when on holiday because it doesn't matter if they get wet. Hope this helps. Regards

I need a womens UK size 5. WHich size do I order on here as I'm not sure if they are UK sizes or USA sizes displayed

There are US-shoe sizes displayed. In your case it would show M5/W7.

Can you put the fleece inserts in these?

I’m not sure , I have bought fur crocs a while back now and they have no strap so when I bought new fur they fitted it , with the strap I’m not sure if it fits so best asking seller