The Building Regulations are mandatory, but the choice of how to comply with them lies with the building owner.
If the guidance in the Technical Handbooks is followed in full then this should indicate that the building regulations have been complied with. However, a designer may put forward other ways of meeting the regulations in the form of alternative means of compliance.
The surveyor assessing the building warrant application will be able to advise an applicant whether or not they need to submit details of alternative means of compliance in writing.
You can download an application form to propose using alternative means of compliance solutions.