Lots of Motor factors can provide the right mix from your paint code and also (I know what some will say but depending what you are touching up) Halfords mix and put it in an aerosol for you as do some other companies. Halfords is done while you wait generally as long as they are not too busy.
Colour fade after a while and often depending on age/colour condition, colour matching is better in my opinion. I remember my white A4 - took two goes and paint code used initially
How small of a pot as you can but the touch up lipstick size touch ups. lol