Woolwich or even Tilbury Ferry? Depends how much you mind traffic and how many km you want to add
I used to do nearly this back in the late '80s/early 90s, but not as far as Dover. Sort of:
A207 (Shooters Hill)
A226 along the N Kent coast
pick up the A2 to cross the Medway etc.
then use Dunkirk & Harbledown etc into Canterbury
my route took me onto A257, you'd want to head for Bridge etc
BUT ... there's now more traffic than there used to be as I found on my 1st annual ride to E Kent from Ips: (year 1 - A2, year 2 - below mentioned bridleway, year 3 - N Kent coast, year 4 (this year) still pending, probably N Kent coast again)
thoughts:
take A258 out of Dover, L to Guston at the top, then eg Eythorne, Aylsham, Bekesbourne (ie along the railway)
pick a crossing of the Stour, then out through Blean and to Faversham, then parallel to the A2 through Teynham, Bobbing etc.
You're still stuck with the A2 and the Medway though.
Probably Gravesend crossing if using this route, then work out something to Stratford.
Beware the N Downs way bridleway - not suitable for cycling (I tried, once)