Q: What happens if Bigwig goes bust?

That's not going to happen! But if it does you don't need to worry, you own the ETF's not Bigwig you can sell them through someone else. Each ETF is registered in your name using what is called a "Holder Identification Number" or HIN. This HIN ensures that we and everybody else knows that you own the ETF's. Transferring the HIN to another broker would allow you to sell them even without Bigwig.