Who should approve changes in curriculum? In the US and UK higher education system, who is responsible for a program curriculum?

For example, if planning to modify the curriculum by adding an uncommon course, who should approve this modification?

Consider that the change is truly uncommon. Can the department head decide about it? or needs the approval of higher officials (such as dean or VP for Academics) or there are boards and committees to decide uncommon changes (for the sake of education quality assurance).
In the US, this is a classic example of shared governance. Typically there will be a well-defined bureaucratic process for proposing a new course. You write an outline and fill out a bunch of forms. Then the other faculty in your department review it. There will also be layers of approval by managers, but the managers are supposed to be reviewing the course for administrative stuff (e.g., maximum section size, which has fiscal implications), not for content (which is where faculty are the experts).

It's important, especially at a community college, to make a course that will be useful to students if they transfer to another school. E.g., I recently proposed a course, Relativity for Poets, at the community college where I teach, and we wanted to make sure that it would transfer to the state university systems so as to satisfy gen. ed. requirements. At community colleges, these concerns may make it essentially impossible to offer a new and innovative course if there is nothing like it in the state university system.

Assuming that a new course does go through all the approvals, it's still not guaranteed that the school will offer it. For example, they could decide that it's too expensive to offer, or there might not be enough demand.