Slightly o/t, I've just discovered a neat little train idea from Southern Railway - their
DaySave ticket. If you buy the ticket at least 7 days in advance, then for just £14, you get unlimited travel anywhere on the Southern network. Or for £28, up to 4 people can travel together.
Currently you can get an advance ticket (with a Network Railcard) from Bognor back to London Victoria for £5, but that's on a specific timed train. The closer we get to the ride, the more the price will go up. The walk-on fare on the day is £18.90, so if you wanted to have maximum flexibility after a boozy morning rehydrating on the beach, it might be worth looking at getting groups of 4 together who are travelling back to the same place, to get a DaySave ticket. This would also be valid for the return from Brighton on 24th August.