Their back sizes are broadly comparable - if anything they run big so if a freya is borderline too big I would go down a back in CK.
Cup sizes. I have their Tempt Me (a plunge) and it is small in the cup. I wear a 30HH in Bravissimo, a 30H or HH in Freya and although I have an HH in the CK I should really have bought a J.
I also have their Tease Me - in a 30H. Was borderline between the H and HH tbh.
I tried their Emily which is a non-padded one in various sizes - I didn't get on with it at all though as it did nothing for my boobs at all. But I would say it was probably similar sizing to Bravissimo
It is worth googling the name of any CK you are thinking of buying as there are a few really good bra bloggers around and they between them have reviewed most of them and include comments on size/fit
I think a few places do - they are made by the same company as Freya and Fantasie. I think their cup sizing is slightly generous compared to Freya as the one I have is a GG and most of my Freyas are an H. Possibly closer to Fantasie sizing which I think is a sniff bigger.
However Leia are absolutely great for returns so you can always order a couple of sizes to be on the safe side if you don't find them in stock anywhere.