not saying you're not gonna make it (agreed too many posts descend into scare mongering)
but
for a day as big as this, i'd want to be in Cardiff with time to spare, soak up the atmosphere and enjoy as much of the day as possible.
even on a clear day a coach from brighton would take presumably 3-4 hours to get to cardiff? i live in bristol and am still getting to the station for 7am to make sure i'm on 9am train (first of day), cos i don't want to be stuck at a station or on a motorway.
good luck to you all making the long journey from sussex. hope it's not an absolute nightmare