If you install a ball valve, won't your water butt refill with mains every time you fill a watering can, thereby removing any capacity for rainwater? Also, you have to turn a tap on to draw water from your water butt so why not just go to the outside tap and fill there when the butt runs out? The effort is the same.
You'll need all sorts of backflow prevention measures if you really do want to put mains into your water butt, especially if you want to submerge the ball valve.
The tank feeds a irrigation system feed by a pump. No watering cans or the like will be filled from the tank, only the pump connected to the solenoid valves connected to the sprinklers. These are all controlled via an automatic timer and rain sensor. Hence the need for this to be automated so the pump does not run dry.
I am planning on installing the valve 3/4 of the way down the tank so there is still enough water to do a cycle of watering but plenty of room for the tank to be filled up by the rain. Hence the question about the ball float being submerged and the effect on the valve.
You should have a low level float switch installed to protect the pump
As for the float valve it can be installed at the top of the tank, you only req a one with an extended float arm
We use keraflo valves as they have a open/close differential in built but I'm talking commercial stuff here