Southern operates a train from Folkestone to Brighton hourly. Tickets cost $40 - $65 and the journey takes 1 h 47 min. Alternatively, you can take a bus from Folkestone to Brighton via Lydd, Camp (Shelter), Hastings Town Centre, Railway Station, and Eastbourne Town Centre, Terminus Road (Arndale Centre) in around 5 h 24 min.
From Folkestone Central train to Ashford International then train to Brighton. 2 h 57 min £45. From Folkestone, Trinity Gardens bus to Lydd, Camp (Shelter), bus to Hastings Town Centre, Railway Station, bus to Eastbourne Town Centre, Terminus Road (Arndale Centre), then bus to Brighton Station. 5 h 24 min £25. From Folkestone drive to Brighton. 1 h 40 min £18.