Can't wait for Google maps to implement cycle routes in the UK, will make training a lot easier.
One option is Keston, Bromley, 30 miles nice ride with lots of green and pretty roads.
The main worry was knowledge of the first day of the trip (never mind the 2nd, 3rd, etc). From Hondarribia to Zumaia takes in 2 hills over 400m, starting at sea-level and climbing without much of a break for an hour or so. Jaizkibel peaks at 455m on the road starting from San Sebastian at sea-level, so equivalent to 3 Ditchling Beacons one after the other.
A London to Brighton day trip via all the hills (will post the route another day) is well worth the effort on a May morning, and good training.