Presumably it depends on whether they are local stopping services terminating there vs. ones that continue on to Bognor Regis/Portsmouth/Bournemouth?
Bognor Regis portions are generally stopping services South of Horsham and, as above, all services running on lines South of Horsham are 377s.
Be aware that Southern do not serve Bournemouth, but do run as far as Southampton Central.
Formations on the route via Dorking are generally 4-10 cars of 377s or 4/8 cars of 455s. There are only a handful of 8 car 455 formations now running on that route, crewed pretty much exclusively by Horsham conductors, who have the joy of locking the rear two coaches out at Dorking for the journey Southbound, and vice versa. Formations via Gatwick Airport range from 3 to 12 cars of 377s.
The newer 5-car 377s (mostly /6s) are quite common on the Dorking route, but are only diagrammed for a small number of services via Gatwick Airport. They do not generally run South of Horsham.
