Mikasa if you are set on avoiding the M25 you could go via Guildford to Horsham then up to Crawley and along A264 to East Grinstead. Although early on a Sunday I think you'll be fine. But this way you've got a back up route.
I was hooked after this event and then booked 3 more!
i ran with my friend who was doing her first 10k today. She managed brilliantly despite being poorly this week. She did stop to walk for a moment with just 400m to go and got a bit tearful with herself. I think it was an incredible achievement and she's up for another event soon!