Work means I often can't get in to Lime Street any earlier than the 19:44 arrival. Provided the train's on time, I always make kick-off. Jump in a cab by going right out the station, not the official rank on the left. The roads are clear by then. Make sure the taxi goes via Mere Lane, and I'm at the ground by 19:55.
At 19:21, the roads will be a lot busier and competition for taxis will be greater, but you should make it easily.