The Act sets up the regulatory framework for  enforcement and allows for the appointment of inspectors etc under the Act. The aim is to manage occupied buildings and to ensue that they are safe while in use (as opposed to during construction or maintenance activities).  The duty to manage it rests with the  accountable person (typically the buildings owner or occupier) or if its more complicated then there should be a principal accountable person. None of this directly effects the trades which you have mentioned. Their function is basically to do as they have been contacted to do and follow instructions from higher up. If they decide not to do so or to cut corners then they could be liable but that is already the case under CDM for example.