If you go to the Golden Guernsey website you will find contact details for the secretary. If you are a member of the GGGS you should have received the annual stud book, which gives you a number of registered and proven males all over the UK (actually none north of Edinburgh, but I guess you are down south.). None listed in Powys or Shropshire though.
Please make sure your girls have a valid CAE certificate if you go to a registered billy.