Holmes, an isolation room within school is another possibility, which avoids the problem of not turning up for detention, but it has to be real isolation, meaning working in silence, and seriously enforced.
I think that ultimately, the sanction has to be the threat of moving to another school away from their friends, or if that doesn't work, then just exclusion full stop. For one thing, parents are much more likely to be 'bothered' if exclusion is a realistic sanction than if it isn't. You will still have children who need to be excluded, and provision ought to be made for them; but on the other hand, if all schools took this kind of firm approach to discipline from the beginning, children would be less likely to get to the stage where poor behaviour has become a habit, along with their belief that they have a "right" to engage in it, meaning that they ultimately have to be excluded. (Of course, this needs to be mixed with genuine pastoral efforts as well, ideally including nurture units where necessary.)
It would also help if all primary schools really took seriously the fact that it is possible to teach nearly every child to read early on by using a good phonics programme, since poor behaviour is strongly associated with poor reading ability.