so what happens when they need to pull up a stretch of track or rebuild/repair a bridge or tunnel? i'm pretty sure they shut the line and divert around adding hours to journey times, but stand to be corrected.
tbh if i was coming from Newbury or anywhere west i'd look at the Southampton route or possibly the via Horsham, and avoid buses. if i was coming from London i might look at the buses to Lewes to avoid the "regular" Brighton bound traffic due to use those buses.
on a serious note, theres a service running Victoria to Brighton with out busses, via Horsham taking 2 hrs, you could pick up at clapham junction.
or get to East Croydon to pick up the southbound trains, then on the buses. there's train to Haywards Heath then get the bus to Lewes which may...