isn't it obvious ?
The build has not been completed hence you don't have a 'completion cert'
It would be chaos otherwise, mortgages are offered on a house that has been completed in law, not a shell which is semi finished.
The building regs list everything, in detail, which have to be complied with in order for it to be signed off as complete.
The insurance you need while its still a building site is called 'contractors all risk' expect to pay £3k+
1st google hit https://www.selfbuild.uk.com/Insurance-For-Self-Builders-faqs.aspx