Oh there is good news and bad news for this!
I know some friends who set up a business on their Teir 5 visa while they were over here but unfortunately they couldn't extend it and they had to return back to aus. The tier 5 youth mobility visa is the hardest visa to extend, and unless you're dating someone from the UK, EU or with a better visa, its not looking good for you
The only thing you might want to look into is the fact that the visa laws for people from the commonwealth are probably going to be changing next year making it possible for us to come here without the same visa restrictions that exist right now. It probably won't come into effect until mid next year though so I hope your visa doesn't expire before then.