Uckfield railway station: The useful rail link is Southern’s Uckfield line: direct trains to/from London Bridge via East Croydon and Oxted take roughly 1 hour 20–30 minutes. Buy an Advance ticket if available; otherwise expect a flexible off-peak single from London to be roughly £15–25, not the obsolete bargain fares sometimes quoted online. The line terminates here: there is no through train to Lewes.
London Bridge station: Take the direct Southern service to Uckfield. It is the cleanest London connection and avoids an unnecessary change at East Croydon; allow about 1 hour 25 minutes. Uckfield station is a short walk from the High Street.
London Gatwick Airport: Do not pay for a private airport transfer. Take a Southern train to East Croydon, then change for the Southern Uckfield service. Allow about 1 hour 20–1 hour 45 minutes door to door depending on the connection, and roughly £15–25 one way. Check the last Uckfield train before landing: this is a branch line, not an all-night airport route.