Usually in British English (Yorkshire, might vary by region), I just hear "It is/was/will be a lark", so I'm not sure if either is particularly common. However,

Usually in British English (Yorkshire, might vary by region), I just hear "It is/was/will be a lark", so I'm not sure if either is particularly common. However, I've definitely come across people using "as a lark", whereas I can't recall hearing "on a lark". I've seen it used in text, but I'm not sure how many were from British English speakers.