There may be a service valve on the pipe work that serves the existing tap , if there is not then its advised to request a full bore one is installed upon the new outside tap being fitted along with an internal drain off valve so to avoid this issue in the future re maintenance.
A competent plumber should be able to locate the boundary water meter & isolate the existing out side tap & then be able to re new your existing stop cock as well , hope this helps & regards Ric