An inspector's job is to gauge the effectiveness of the school, which includes the learning that goes on in the lessons. It is not their job to impose any official or personally preferred methodology. That is why you won't find an Ofsted-recommended format for a lesson plan.
Lesson planning varies enormously from school to school and even, sometimes, within schools. That is only to be expected because schools and their pupils are not all the same and one size is unlikely to fit all.
Often teachers prepare lesson plans when they are being inspected even when they do not produce similar written plans on a routine day. When they do this, however, it is not at inspectors' request. I have inspected schools with written lesson plans in every imaginable format; and I have also seen exceptionally effective lessons where no written plans were proffered.
Of course, not having a written lesson plan is not the same as not having a plan at all. If a teacher is unprepared and the lesson is rambling and purposeless, then it is unlikely that pupils will be learning much.
Because inspectors are more likely only to be in class for parts of lessons rather than seeing them from start to finish, then, where there is written planning, this can be helpful to inspectors in seeing what has gone before and what is expected to follow. Where, for example, inspectors are focusing on how well particular groups are faring, then it can be helpful for them to be able to see how the teacher or support staff plan to match work to their needs or capabilities. If there is not a written lesson plan, or the planning is unclear on this point, this may be the sort of question that inspectors may want to follow up with you when you see them for feedback on the lesson.
If your school is one of those that decides to produce plans for inspectors' benefit, it is helpful to include a note of the number of pupils in the class and setting arrangements, as this is information inspectors are required to complete on observation forms.
Selwyn Ward has been an inspector for 15 years, working in primary and secondary schools. The views expressed here are his own. To ask him a question, email him at firstname.lastname@example.org.