Under options/keymapping/other, you can set the command Disable UI to any key you want.
Also, if you opt to use demos, the UI will automatically be hidden in demoplayback.
CoH Codex : Demo Models/FX/MOVs : Demo Info
Arc 111022: "Doctor Geist and the Scientific Method"
I made this pic for someone a while ago.
Link
Hope that helps 
I'm not sure if this is the right place to be posting this but I'm just wondering if there's any way to hide all UI for a video I'm going to make. Any help is appreciated. Thanks.