Most Woolworths do some varieties.
Many sweet shops/newsagents also stock it.
I did once find a Milka hot chocolate drink in a cheap shop but it tasted bad.
http://www.chocolat.com/index.asp You can buy Milka bars here. I'm going to be ordering some for my Dad. He had a mad passion for Lila Pause bars in the early 90's and then they took them off the shelves and he has been depressed ever since 
My friend always brings me Milka from Germany, as it was my wish on her quiry what would l like as a present from Europe.
They used to sell them in Asda and I was happy to buy them every week untill they were gone. Now I am looking for them every where and couldn't find it yet (Manchester area). I found that eBay sends Milka as well.