You'll need a plumber for the Shower and Sink installation and an electrician for the loft light. Most decent plumbers will have progressed into heating and gained Gas Safe qualifications. Although, some plumbers do specialise in bathrooms and don't bother with boilers or gas safe qualifications.
An electrician would normally have a governing body like NIC or Stroma.
All trades can be Safe contractors as long as they have a health and safety policies in place within their company structure, normally required for commercial contracts and not domestic.
Sole traders are generally allowed to use common sense procedures to protect themselves from injury in the workplace.
Hope this helps!