Hello! You need to consider where UI and UX come in and why. Typically, by the time UI/UX is designed and then reviewed a range of activities must've taken place, e.g.:

(1) Someone defined the proposition, product or service which the UI/UX is supposed to satisfy.
(2) Someone created a project around the UI/UX design and implementation.
(3) Someone created a blueprint for integrating the UI with existing systems and thought of ways to streamline UX across products or services.

If you provide services in any of the above, they will constitute a value-add to UI/UX. One obvious angle to play, if you're doing reviews, would be to try and determine why UI/UX fails to achieve what your client wants. This presents a broader methodology improvement opportunity for your client - a valuable lesson.

More than happy to share some of my experiences delivering UI/UX changes as parts of digital transformation. I've also authored standards and guidelines for UI/UX design for large corporations so know what can go wrong and what would sell as a complementary service to your reviews.