I assume that you are taking on the role of “developer”? As I understand it, the 3 (triennial) and 10 year (decennial) elements of such insurance are mandatory but the 1 year element can be replaced by the developer withholding 5% of the contractual price of the works – so make sure you have a retention clause written in to your contract with the builder.
Normally the builder is the policyholder for the 1 year insurance and the developer is the policyholder for the 3 and 10 years parts although the builder can also be the policyholder for these.
I believe the 3 and 10 year insurances are mandatory because they affect the habitability and structural safety of the building. You would have problems selling on without these in place anyway.
To protect yourself further ensure that you employ a qualified “project manager” or similar to inspect the works regularly or retain your architect to do this, if they are independent of the builder.
Also, ask yourself a question – would you go ahead on a building project in the UK without an NHBC warrantied builder just to save yourself a few bucks initially?