Why navies plan even for wars with friendly powers

Did the Royal Navy maintain plans against friendly nations in case they became enemies?

Yes; staffs planned for both likely threats and low-probability but catastrophic contingencies, including conflict with powerful friendly states.

A friendly state can still be the most demanding opponent a navy could conceivably face. Britain and the United States studied each other because neither could readily overwhelm the other's industrial capacity in the way it might a smaller power. The explanation distinguishes likely threats from unlikely threats whose consequences would be severe enough to require immediate preparation. Writing plans and conducting the necessary studies also kept the Royal Navy's large interwar staff trained for wartime work. A low threshold for preparing contingencies therefore produces many plans without establishing that the navy expects to fight every state covered by them.