This seems like it's begging for a test case to strike this down on humanitarian grounds. I can't imagine any western judge of any serious level (possibly some county judge in West Texas or something) would take seriously the claim that this wasn't a violation of basic human rights (in US terms, the 14th amendment, though I assume similar principles apply elsewhere), and that such contracts were unenforceable. IANAL and I'm certainly not a limey lawyer, but our notions of justice on both sides of the pond share a lot of DNA.