St. Agnes is not a point; it's an island.
According to Wikipedia, the southernmost part of St. Agnes is known as Wingletang Down, and the southernmost extremity of Wingletang Down is called Horse Point. The only possible answer to this question (if we take Wikipedia's word) is Horse Point.
There is also a small, apparently unnamed (and almost certainly uninhabited) island to the east of Horse Point, which extends further south than St. Agnes. The southernmost point of this island (which, similarly, is almost certainly unnamed) may be the UK's most southerly point – or it may not (there may be other islands I haven't noticed).
Of course, no one could really be expected to know all this (unless perhaps they'd just been to the Scilly Isles for a holiday). The question should have asked "What's the most southerly inhabited island in the UK?"
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