Navigating

2D Viewport

Panning can be done by dragging with MMB.

Zooming can be done using Wheel or NumpadPlus/NumpadMinus.

Gizmos

Next to the Sidebar region at the top, there are gizmos that allow panning and zooming more comfortably when e.g. no mouse wheel is available.

View Menu

Also see Navigating in the Image Editor.

Frame Selected NumpadPeriod

Change the view so that all selected UV vertices are visible.

2D Cursor

Just like the 3D Viewport, the UV Editor has a Cursor that you can jump to (View ‣ Center View to Cursor). It can also serve as a pivot point and a snapping target.

To change the Cursor's position, either press LMB with the Cursor tool selected, or Shift-RMB with any tool selected. You can also change the "Location X/Y" fields in the View tab of the Sidebar, in either relative coordinates (0 to 1) or pixel coordinates. In both cases, the lower left corner of the image serves as the origin (0, 0).

You can press Shift-C to move the Cursor to the center.