The cycle path alongside the A23 out of Brighton isn't bad and has the BIG advantage of avoiding Devil's Dyke. You then pick up our normal route at Pycombe. After Crawley I shall be heading over my usual cross country route (Copthorne, Lingfield, Edenbridge, then one of Hosey, Toys or Ide Hill depending on my legs, then Polhill and so on) rather than up the A23 via Croydon which can be busy on a Saturday morning.
Ant - I'm happy riding on main roads, but I wouldn't recommend the A23 from Brighton to Crawley. It's a very fast dual carriageway and quite nasty in places - especially Handcross Hill which will be lethal on a bike. We normally run parallel to it, to the west. And it's a cracking country route - apart from The Dyke that is.