I think its too far gone for trim restorer tbh.
I would save the time and money and get a paint can. If you wanted to stick with the grey, check the Halfords car paint cans (the matched ones) and see if you can find a grey that suits you. You wont need any clear coat on that unless you want the super gloss finish. If black, the halfords own brand blacks are decent to use. Make sure to sand a bit with 2000/2500 gritt sand paper and possibly use a plastic primer for best result.