When you say "Imperial Medals" I assume you are referring to the various orders, such as the Order of the British Empire, Order of the Bath etc, and not campaign or gallantry medals. In most instances recommendations for these awarded are forward to a government committee for approval, or otherwise. These recommendations are forwarded to the monarch for approval but I think in most cases this can best be described as a "rubber stamp" job. There are exceptions. Appointments to the Order of the Garter (and its Scottish equivalent the Order of the Thistle) and the Order of Merit are made by the monarch personally, without reference to the Prime Minister, as are appointments to the Royal Victorian Order which is for personal services to the monarch.