这个句子没错.
most of the time 是名词短语,引导时间状语从句.后面直接跟句子,不能加when的.
类似的用法还有:every time/each time /the first time /next time +句子,都是时间状语从句.
如:The first time I saw him,I began to love him .
Next time you came here,please come to see me .
which 引导的从句是非限制性定从,which指代前面整个句子.
句意:我们在英国的大多数时候,天都在下雨,这太令人失望了.