In my experience, lambs do not thrive on pasteurised, homogenised cows milk. As in, they die.
I have reared lambs on raw Jersey milk (so gold top - but not pasteurised and not homogenised) straight from the cow, but they need a lot more of it than the powdered stuff, so much so that when very young I've had to feed two extra feeds a day as their stomachs aren't large enough to take the quantity they need in 3 or 4 feeds.
You could certainly do it on an ad lib system, but definitely not homogenised. I'm not sure about pasteurisation; personally I'd want it a as natural as possible,
There are also plenty of people who rear lambs on goats milk. But again, straight from the goat, not processed.