If you are going to charge a living wage rate, then you should become an expert. A PAT test is not just pushing a few buttons on a device and writing a label; you need to keep records, know about electricity, be sympathetic to the many different types of equipment and environments you will encounter, but more importantly you should have a thorough understanding about the safety requirements that you will be testing against.
Performing in-house tests may require less thorough knowledge, as typically the tester knows the equipment and works with it every day, they know how to test it and what the typical results should be. There is likely to be quite a limited array of equipment types for them to test, too.
As an independant tester, you may need to test a vast array of different types of equipment in order to make a living at it, and you would need to know about each one as performing the wrong test could destroy it, or not test it thoroughly enough.
Just a few thoughts to consider.