Such(表语）// is（系动词）// the mess（主语）// Britain finds itself in（主语后置定语）// that even jettisoning a powerless prime minister is not really a step forward（结果状语从句）.

还原为正常语序为下面句子 

The mess（主语）// Britain finds itself in（主语后置定语）// is (系动词）//such(表语）// that even jettisoning a powerless prime minister is not really a step forward（结果状语从句）.

主语+系动词be+such that(结果状语从句）结构中，表语“such”提前句首倒装结构,类似于像相面倒装结构例句如下

Such is the size of the problem that these arguments are purely academic. 问题的实情在于这些论点纯属空谈。

Such is the sensitivity of the information that only two people are allowed to know it.  

这信息极为敏感,只允许两个人知道.