My suggestion is 10lb+ monofilament fishing line, simply because braid is banned as a mainline on most of my local waters. I’m currently using 15lb Daiwa Sensor monofilament – It came free with my reels! This is available for about £5 for 300 meters.
The minimum I am allowed on the local specimen lake is 10lb.
I would suggest that you check the local lake websites just in case, but 10lb line would be fine to begin with.