It is a legal obligation for the homeowner (self-builder) to be on top of health and safety on site and there are procedures in place for you to prove that to the authorities. However, it is not mandatory to take out self-build insurance. 

Insurance is however necessary as a safety net that will cover your costs in the event of a claim related to your build or renovation project. In other words, what insurance will do is protect you from the unforeseens and the many risks associated with construction activity, including trespassers getting injured and filing a claim against you, or the theft of tools or sanitaryware waiting to be installed. In fact, most lenders will require that you take out an insurance policy if you go direct labour. 

Know that if you are accessing any kind of third party financing it is very likely that some form of structural warranty will also be required for your build. The warranty provider will likely undertake an assessment of the construction drawings followed by a schedule of inspections throughout the build. Mortgage stage payments are often only released when the inspector reports satisfaction.