With the numbe rof people I've recommended the site to, your UK users could well go up :-)
I have used a lot of custom prizes as rather than rewarding the kids with more toys, we have focused on 'family time' as being the reward - trips out, activities we can do as a group so all the kids have to earn enough points to get the treat - encourages team work as they remind each other of what needs doing!
This should work well at weekends when we have my partners 2 kids as well as mine, but I have given my son a separate account for his weekday tasks and would like to reward him separately from the others in relation to this. I suppose I could set up another account altogether to keep the weekends with all 3 of them separate from weekdays with just my son.