Brighton's great and easy. Tons to do besides the beach, lots of art and music.

if you want to really see the coast, take a train to eastbourne, then quick cab to Beachy Head. The Seven Sisters is glorious walking (if it's nice out -- no guarantee).

https://tripsget.com/day-trip-from-london-to-seven-sisters-country-park-cliffs-2/

brings back happy memories of racing the south downs relay when i lived in the UK in the 90s. These cliffs are the starting point. Turn your back on the sea, follow the acorns, and 100 miles later you'll be tired and in Winchester.


......and Brighton is a great suggestion for a British seaside town.