"A clearly-structured and well-developed argument" is a core component of the marking criteria used by the English faculty. Without good structure, the content and style alone will not be sufficient to gain the highest marks: "a concentration on knowledge at the expense of a clearly-structured argument is likely to pull a candidate down".
A well-structured essay depends on a structured approach at every step, from analysing the question to the final editing. However, in reality, the process of essay writing rarely conforms to the commonly cited model of read→plan→write, and there are many ways in which structure can be incorporated into the process other than by having a formal planning stage at the beginning. Structure can be introduced at different stages, according to your working preferences, whether you are the kind of writer who plans every paragraph before it is written, or whether you write to develop your ideas and impose a structure on them subsequently.
This resource offers a range of strategies which suit various types of writer, and can be incorporated flexibly into your working habits and preferences to enhance the structure of your writing.