The Building Regulations 2010 (as amended) are a set of mandatory functional requirements established in law to protect the health, safety, welfare and convenience of building users and people who might be affected by buildings or matters connected with buildings. They also define the thermal performance of buildings and building works.
The Building Regulations are not concerned with aesthetics (what the installations looks like) or the quality of the installation so long as it does not adversely affect the thermal performance or the health, safety, welfare and convenience of building users and the general public.
Therefore poor finishing of an installation such as untidy sealant, large trims or uneven finishes although not satisfactory to a householder may well not breach the building regulations. Such issues are likely to be contractual issues that should be taken up with the installer at the time of the installation.