I'm not from the West Midlands, But are you sure they use Aurmoured Land Rovers as ARVs. As far as I know ARVs are normally Volvos and BMWs. Also they probably use a Volvo V70 as a traffic car and general patrol car.
Load the editor and click the modifications menu at the top. Click unpack and look for the file. But I don't think it will let you unlock the original game files.
Right. You need to use DDS Converter which can be downloaded from the Download centre and convert it to .jpg and then edit it using Paint or something similar. Then you need to convert it back to .dds. Or alternatively you could download Paint.NET and use that because it can open .dds files so they won't need converting. Sometimes the converter doesn't work properly so I would recommend usng Paint.NET.
I don't think you can edit the original models. You would have to unlock the file in the editor but I don't think the editor lets you unlock the games original models