OK, here's the story... We were wondering how many lessons a UK secondary teacher actually teaches each year (and thus: how many starters they need)
The obvious answer is 190 (teaching days) x 5 (lessons a day) = 950... but, that doesn't take into account PPA lessons (even TM4T teachers don't do a starter in PPA lessons). So that brings it down to 190 x 4 = 760...
And, that doesn't allow for 'repeats'. If I'm teaching a lesson on Pizzas to 7R in Period 4, and Pizzas to 7S in Period 5, they generally get the same starter. Not always, but generally. And it doesn't allow for 'double periods'; my A Level students don't get two starters in their double lessons. And it doesn't allow for sports days, work experience, school trips, or any of the other curriculum-disrupting enrichments which are scattered through our student's lives.
So: we did a highly scientific survey of UK teachers - approximately 3 of them to ensure a balanced and statistically valid result - and decided that 642 was the answer. I'm sure that the Department of Education can come up with a better number, but they'll probably spend £1/4 million doing so, and it will still be wrong.