There is a tenable case for James I of Great Britain, although logically we would have to move the next 5 monarchs as well.  The arguments could be that we had a comparable discussion in relation to the kings of Spain, they should be described as such following the presonal union of Ferdinand and Isabella, even though Castile and Aragon were not fully united until some time later.  The title King of Great Britain was used at the time, e.g. Charles II was proclaimed as such by the Scots after the execution of his father.