Put a group of people of different ethnicities, cultures and classes in a house together and ask them to co-operate and it's more than likely they will. People are people. The group assembled in Bradford were hoping to get along with one another- so they did. There was a certain amount of friction. A deeply religious Muslim thought it would be acceptable to spend hours hanging out at the mosque away from his housemates, a retired policeman caused upset because he assumed the others would take it in good part if he slung racist terms around in a jocular manner; both were confronted about their behaviour and adapted. Two other housemates entered the experiment with racist views and left without them (or so they said).
The things that keep us apart are mostly add-ons- and people of goodwill don't need much prompting to relate to one another at the level of our basic humanity. Where it becomes trickier is with people en masse. Groups of people- communities, nations, mobs, religions, political parties- are so much stupider than individuals.
The housemates were all British citizens- chosen after they'd failed the general knowledge test administered to people seeking citizenship. Most people who take it cold do fail- around 90%. There's a version of it on the website above. I just took it and failed too. Most of my answers were guesses. Either they're asking the wrong questions, or nationality has little to do with a shared corpus of information.