You can but you will find a steriliser and cleaner works better because it cleans as well. If your keg is perfectly clean and just needs sterilising then this doesn't matter.
I use Milton pretty much for sterilising all already clean stuff. Anything that needs a bit more work I use the Youngs cleaner and steriliser or soda crystals to get it properly clean first, then a short soak in a bucket of the milton solution before a couple of rinses with water.
