On Thursday morning, Britain looked over the edge into one of the dark places to which modern public life has been heading. Having looked and seen, Britain now needs to learn and step back.
which 指代 one of the dark places
Heading = head to
Having looked and seen 表示 look和see 是发生在主句谓语的前面，因此是使用having加过去分词，而不是直接过去分词
