Thanks both of you; excess is over £250, and I only got worried after reading about people getting charged >£100 for a small scratch on a plastic hubcap....
Will try with the toothpaste, and then with the polish if I can get my hands on some. Also found some similar light shades of permanent marker at home which I might use on any specifically white bits..
One tip I also found online, is that the rental places (in the UK) specifically check the left side of the car (especially the left front wheel), as that's where most damage happens, usually from foreign drivers (and that's where I made the scratch). Someone mentioned to switch the front left and back right hubcaps and that it won't get noticed then. Though the hubcaps are screwed on and I don't exactly have the tools for changing them.. Hope I'm relatively fine now though, thanks for the tips and I'll post an update later