I've always had fond memories of the traditional FNRttC Whitstable route which involved the delights of Croom's Hill at Greenwich, Shooters Hill, the urban misery of Welling, and the cycle path over the A249 to drop down into Sittingbourne amongst others.
Seeing as I haven't run any rides into Kent for a while, I've put together a "best of" route to Dover, which incorporates the above, along with the tank museum in Chatham, a stunning ridge top ride along the top of the Downs, and then a 10 mile continuous stretch of road where you don't have to think about whether or not you need to turn left or right, with a lovely swoop downhill and there's only 1 crossroads to worry about, before passing an historic WWII airfield and popping out above the cliff tops between Folkestone and Dover. Depending upon time/hunger pressures, we can make a detour to the Battle of Britain Memorial site, before the long, fast descent into Dover.
As it's just over 90 miles, I'm running this as a night ride, so leaving Hyde Park Corner in old London town at midnight on Friday 9th August, stopping off at the 24 hour Asda in Sittingbourne after 48 miles for a break, and then breakfast in the middle of Dover. Usual rules apply - no-one left behind. The approximate route is shown
here.
You can cycle back, although trains are available.