Hey ! i'm creating a mod and i need to create the UK but the Uk steel exist... i need to modify this xountry but i need to create a new Uk... Help!

When you make a new mod, you can't edit a country through this website. What you can do is make a new idea or focus tree (whatever you neccessarily want to make) and put the tag as ENG (the tag for the UK). I think it will apply the changes, because hoi4 will read the new focus tree or idea or whatever as apart of the game, and give it higher priority, in turn changing the focus tree.