20mm galv conduit and it easy to put a thread on it. Not sure what you
intend using it for, so difficult to determine what it should screw
into.

+1

You have not made it clear what you intend to use the conduit for, but other than electrical fittings it hard to see what you would screw the conduit into. For most electrical work they are screwed into conduit boxes or couplers, there is a very limited number of other fittings a few which could possibly be used for construction purpose such as inspection bends and inspection tees but these are rather expensive.

Richard