Hi, What do people use as an overflow for a water butt ?? Thank you in advance?

Usually it's a soakaway or French drain. (Why it's called French is beyond me). You dig a pit around 3 feet deep (deeper if your soil doesn't drain very well) and put around 2 feet of pieces of rubble in it. Add a length of drain pipe to clear the surface and fill the rest of the hole with soil. You can add a fancy gully if you wish, but water isn't fussy.

A drain or a channel.