Equal pay is an easy concept when you are talking about waiting tables or stacking shelves.
As someone else has pointed out, quite a lot of jobs will have pay bands, and the employees will be remunerated depending on qualifications and experience. Even though both are doing substantively the same work.
The place I work operates a pay band system based on time served. The person whom I share an office with, who happens to be female, has been around longer and is paid a few thousand more than me for no other reason than she has been here longer. We do exactly the same job. Should I launch an equal pay claim?