To be fair, with a place like Brighton, there is never an ideal weekend. True, thousands are affected on home match weekends when one of the lines is closed, but even on international weekends, thousands of other people still use the trains to do other stuff just because there is no football...
Back in those days, we didn't have a government quite so intent on screwing it up so ideologically (by believing they could smash the unions) or deliberately (by believing they could smash the paying public for not supporting them in trying to smash the unions) or completely.
They make a detour via every station, so takes yonks. Bring a flask and a packed lunch.
The 700 isn't too shabby - all along the seafront. You can then pick up the 25 from Churchill Square or Old Steine. Either way, allow 40 minutes to get to Brighton.