In reply to hanspan85:
I cycled to Brighton from North London using google cycling directions when it was released on the phone app to test it out, which I think used quite a lot of route 20. Going through Croyden was pretty miserable, it sent me on a bit of a forest trail which was unrideable on a road bike, and there was a bit which was meant to be a cycle path next to a duel carriageway, but it was majorly overgrown and I had to get off and climb over the plants for a bit. There were some hills but nothing major.
Now I have a bike satnav I think I'd plan a nicer route based on Tom's route linked to above, but part of that trip was seeing what Google thought were good cycle routes.
I came back on the train without booking (in September, on a Sunday).