Instead of heading south, try the Kent Downs to the east of London. Follow the lanes through Pratt's Bottom and Shoreham (can you resist the pubs?) before a climb up Fackenden Lane.
Once you reach the highest point of the ride, you're rewarded with a speedy descent into Knatts Valley, swapping the narrow lanes for sweeping views across the valley floor. Jump on a wheel and hang on.
Eynsford has a good halfway café just before you cross the river, and a castle you can cycle round. When you're ready, head back to London the fast way and save some matches for the climb back up the other side of Crystal Palace park.
Thanks to Martin Strotton for this one.