You will need to ask National Coach of the arrival time.
Hello Gloria . The distance you will be travelling is about 300 miles I cannot believe that you could make such a booking without knowing the timetable. As you have not even told us which ship you intend to board we have no way to know your latest boarding time. Travelling time has got to be in excess of six hours I would think. Sorry Gloria but you and I have too little information to guess answers. Heathe.
Why not use the dedicated cruise coach which will take you direct to the ship in Southampton avoiding London. if you are travelling independently on National Express, the ship will not wait for you. You do not say what ship you are on, but there are two P&O and one Fred Olsen ships leaving that day. P&O have a direct coach link from Bradford leaving at 7am arrives 13:30-14:00 and Fred Olsen have something similar but I don't know the details.
As far as I am aware the ship will sail without you. This is because coach services to the ship are by independent companies and are not organised by (or the responsibility of) the cruise company. The fact that they may be promoted in the cruise brochure doesn't make any difference.