You shouldn’t need to match the number plate, there should be a decal specifically for the number plate. Most Tamiya & Revell car kits come in left hand drive so matching a uk plate wouldn’t make sense anyway. Also don’t forget your printer will not necessarily give accurate colour matches.
Tamiya also do a decent range of spray cans specifically for plastic model kits, they’re a cheaper option than buying an airbrush but very much for the large coverage. Thin coats, left to dry, then repeated are best. Finish with a coat or three of clear gloss varnish on the bodywork.
Smaller parts are best done with a fine brush – 0 or 00 size – while still on the spruce.