In London no one nods. It must be because there are so many bikers you would ruin you neck. I'm looking forward to touring Europe next week, driving on the right all bikers wave instead of nod. :-)
I'd always stop for a fellow biker, although around here we get a lot of youngsters on stolen bikes (broken fearing, no lights, no exhaust, no gear, etc). I would gladly leave one of them at the side of the road.