According to Philip's road maps it is 381 miles.
About 320 miles
Dover Marine ended in 1994.
Approx. 833 miles
go to minecraftwiki.net and search far lands
Dover Harbour railway station ended in 1927.
Dover Town railway station ended in 1914.
As far as I know there is no ferry service between Dover and Belgium anymore. From Dover you have a ferry to Calais in France.
22 miles
about 390km
About 280 miles.
By road, 77.2 miles.
About 100 miles