Ok, this is getting a little confusing now. Can you still pay nothing, or is it 6 pound minimum? If not it shouldn't really be considered a donation anymore should it? Why do people have to find yourself at the event to make a larger donation, why can't they just do it through the website, have it like buying multiple items at 1 pound each. Say you want to donate 10 pounds you pre register and add on 4 items. It's a bit more of a 'proper' way to do it as the rider won't feel under pressure if he intends on only donating a couple of pounds more.
So the people who donate 10p more will have the same stake in the raffle as someone who's donated 10 pounds more.
I also think the 2nd wristband / raffle isn't a great idea as you can see who's donated more and who hasn't, if you're out to guilt people into giving more money then fair enough but i don't think that's the goal. I think you're trying to encourage people to donate more but you're just inadvertently going to alienate the people who could barely afford attending the event anyway.
The way I'd do it personally is charge 6 pounds for the event via tartybikes. No more no less. 10 pounds if you don't pre register is a good call. And then encourage people to buy regular raffle tickets whitest you're there. That way the more money people donate the more chance they have of winning, and if they want to turn down donating more money this way then they can do without anyone really knowing.