We already have a Platform 1, which will presumably be extended into the Old Station if that ever happens; the other one would become Platform 0 - which will make it one of the few but growing numbering systems that starts in the right place!
Renumbering 15 to 14 would break the convention that London-bound platforms are odd whilst Exeter-bound are even - which might confuse some people. A hang on, 0 is an even number!