Like 500 said. There is absolutely zero legal requirement (under both the UK ANO and EASA FCL) to do any ground exam before first solo. It's often a school requirement (and that makes good sense), but that doesn't make it a legal requirement.
So once the instructor feels you're capable of taxiing around on your own, he can sign you out to do so. In general there won't be any practical value to it, but I can imagine students picking up the aircraft from a far-away apron before a lesson, or bringing it back after a lesson, while the instructor goes on with the next lesson.