From London, we’d like to take a day trip by train, to the English coast in October. We want to visit an area with restaurants that serve seafood, has scenic views of the coastline and some historic battlefields, churches or homes. I’m looking at a map and thinking of the Dover area, but I’m sure some of you well traveled posters will be able to suggest lots of wonderful towns and villages. Thank you for your help!
Dover area (Kent) is good. Dover itself has the castle, and nothing else worth seeing.
Elsewhere in Kent you have Canterbury, Leeds castle, Roman forts at Reculver (click here) and Richborough (click here). the latter next to the town of Sandwich (click here), not just famous for its Earl inventing a fast food.
List of National Trust locations in Kent: https://www.nationaltrust.org.uk/days-out/regionlondonsoutheast/kent
If you venture a bit further you have Hever castle and Rye.
BTW, Dover isn't the East Coast, Kent counts as the start of the South Coast. The East Coast is everything from the Thames up to the Scottish border: Essex, East Anglia, Lincolnshire, Yorkshire, Durham and Northumberland.
For all you want I strongly recommend Northumberland, but that is a full day's travel from London.