According to Philip's road maps it is 381 miles.
About 320 miles
Dover Marine ended in 1994.
Approx. 833 miles
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.
Dover Harbour railway station ended in 1927.
Dover Town railway station ended in 1914.
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22 miles
about 390km
About 280 miles.
By road, 77.2 miles.
About 100 miles