What type of ball valve would you fit in a domestic water tank in a loft and which should you not fit

Use any ball valve approved for potable water use. You probably want to use valve that is easy to turn On and OFF. That would be 1/4 turn ball valve with lever handle.

What size is your incoming line? Ball valves are pretty much all the same but must match the incoming line.

What you see in Milo's pic is one for threaded connections. They make one for sweated connections as well. There may be valves rated for both gas and water, but you want one that works for water.

One other option that's available for valves is a small drain plug. It's put in usually so it drains the outlet side of the valve.