This code uses the tool's ProgId:
pUID.value = "esriCore.SketchTool"
It has been my experience that using the ProgId requires the command or tool to be on a toolbar or menu somewhere.
If you use the actual class GUID, then the command or tool does not have to be on a toolbar or menu:
pUID.value = "{" & yourClass.ClassId & "}"
The code above is .NET and assumes the command/tool class is in your code project. You can also just use the hard-coded GUID (you'll have to do this for built-in tools and tools whose code is not in your project).