Why are houses so cheap in southall (west southall)? its close to heathrow and london. ?

Because Southall is almost 100% Asian (it got that way through being so close to Heathrow Airport so it was easy for immigrants to settle there, and once the first few did it the rest followed them) and that puts almost everyone else off buying a house there because people like to live near people like themselves. And Indians and Pakistanis are very good at driving prices down as far as they can. Don't get me wrong - I'm not being racist, it's just a fact of life.