I was at Stonehenge when I was a kid so my memory is a wee bit fuzzy. I don't remember the channels, I'm not sure how deep they are or how much water they would hold. It's an interesting idea though. That been said, I doubt England even back in the day would have the temps to make ice thick enough to the slide massive stones on, unless the water was very shallow. Walking on ice, even 3 inches thick is not sufficient to carry the load of an adult. In Canada, we have ice roads, and unless the ice is at least 12 inches thick, it will not hold vehicles weighing up to 2000 lbs, the average car. You must have at least 15 inches to carry a cube van and even thicker ice, up to metre thick to insure a heavier load on big rigs will not go through the ice.
Most of these roads are on deep bodies of water, lakes, and in some cases moving water, such as rivers.. The temperature must be below -10C or below for a sustained period before it becomes dependable, the lower the temp the faster ice will thicken. If the ice is covered with snow, it's insulated and is not trustworthy or if it's cloudy it has become degraded and it's too dangerous.
I think I read a long time ago, so I can't verify this... that the channels were thought to be used to ferry the stones on rafts. But, if the mean winter temperature was cold enough for long enough, it could be possible the stones could have be slid along shallow ice wells. But that would mean the temperatures were horrible mid-winter for the ancients, and maybe they were.