Be aware that Home bond doesnt cover extensions.
You want a builder who knows what they are doing, who is properly insured and has a checkable history and references.
Yes you may feel that you have more options with a soletrader if he goes out of business, directly responsible etc, if he has no money or assets it will mean absolutely nothing. You also can not check the financial status of a soletrader.
A company can not just go out of business, yes they may shut their doors and be just as uncontactable as the sole trader but there is a lot of legal compliance, cost and paperwork involved in liquidating a company. You can check a companies history and financial status, yes they may be a year old in the CRO, but it is a check point.