Brighton is a mere 3 miles further from you than Bath, but a quicker drive according to Google Maps.
Google Maps clearly has a lot to learn about the utter shitfest that is the A27 it seems.
I've just noticed we're away to Bristol Rovers in the League Cup and it coincides with the boy being...
It is possible to go direct (or at least it used to be) on a First Great Western train that runs direct from Brighton to Cardiff, stopping at the major stations along the coast.
Not sure you could use it in both directions for a day trip, but it might be possible to take it in one direction.