The British value thorough preparation but expect it to be carried out without conspicuous display. The ideal is to be well-prepared while making it look easy—or at least not making a visible production out of the planning process. Drawing attention to how hard you worked on the plan, or how elaborate your preparation was, does not increase your credibility. Competence in planning is demonstrated through results, not through visible effort.
A concise, clear plan is more culturally admired than an exhaustive, elaborate one—even if the exhaustive plan involved more work. The cultural expectation is that planning is something competent people just do, and that the results should speak for themselves rather than being accompanied by a display of the effort that produced them.