Heathrow Connect operates a train from Marble Arch to Southall every 20 minutes. Tickets cost $10 - $11 and the journey takes 15 min. Alternatively, London Buses operates a bus from Marble Arch to Southall every 30 minutes. Tickets cost $3 and the journey takes 38 min.
Yes, there is a direct bus departing from Marble Arch and arriving at Southall Park. Services depart every 30 minutes, and operate Monday, Friday, Saturday and Sunday. The journey takes approximately 38 mins.