Toughness: 6 out of 10: a couple of steep hill climbs
9.46 train from Victoria (9.53 Clapham Junction, 10.03 East Croydon) to Lewes, arriving 10.53, changing there - be quick! - for the 10.59 to Southease, arriving 11.06.
I have been meaning to try this idea for a walk for some time. The idea is to do the shorter version (option a) of the Southease to Seaford walk (ie omitting the loop to Rodmell) in the morning, getting to Seaford for lunch after 7.6 miles. There is then the possibility of a sea swim on Seaford’s beach, before carrying on in the afternoon (optional - you could simply spend the afternoon on the beach) over Seaford Head to Cuckmere Haven (where another swim might be possible), and then on to Exceat for tea and a very regular (every ten minutes or so) number 12 bus back to Seaford. Seaford to Exceat is 6.2km (3.9 miles).
Directions for the morning part of the walk are here. GPX files are here. For the afternoon directions are not really necessary: follow the cliff path to Cuckmere Haven, then follow the riverbank inland to Exceat. But if you want directions, see paragraph 135 on page 12 of this document (you only need page 12). For GPX click here.
There are various lunch options in Seaford, including the Trawlers fish and chip shop by the station and the nearby Plough Inn near the church. Tea at Exceat is at the Cuckmere Inn or the Saltmarsh tea room (cross the road bridge and follow the road for 400 metres to the Seven Sisters Visitor Centre complex).
The number 12 bus runs to Seaford (taking 15 minutes or so) every ten minutes (until 7pm or so: half hourly after that) from both the Cuckmere Inn and Seven Sister Visitor Centre.
Trains back from Seaford are at 25 and 53 past the hour till late. (Note that all these trains now offer London connections via Lewes: there are no longer two evening trains where you have to connect via Brighton). T=2.26