You need to change the document properties of the default part template files to use Imperial units.
It's typically named part.prtdot and is found in a path similar to this: C:\Program Files\SolidWorks\data\templates\Part.prtdot
Open that file as you would any other part, then take the following steps:
Select the Tools menu -> Choose Options -> Select right tab, "Document Properties" -> Select Units -> Change to IPS and make other changes as needed -> Save As and change file extension to Template (prtdot) -> Save back to original path.
You may be able to just save the template file you've opened rather than doing a "Save As," but I'm not about to try it and break my templates
