I find this hard to credit, but I started the 5:2 diet thing with my first "fast" day Tuesday last week. (I do Tuesdays and Thursdays, so I completed my 4th fast day yesterday.) Weight loss seems to definitely be over 2 kg, as I started on 89.something (I think it .5 or above, as the rapidly approaching 90 kg line is what finally convinced me I have to stop putting on weight. But I didn't write it down.) This morning: 87.1, after breakfast.
I have taken no exercise of note in the period.
A weight of 83.5 would get me into a BMI a shade under 25. (I am currently 26). My wife tells me I was 82kg when we married, although I honestly can't remember.
82 or 83 probably sounds a reasonable goal, and then I might watch what happens if I only do one day a week on 600 cal (as Michael Mosley says he found that was all that was necessary after taking off the weight.)
This does seem a very impressive way to lose weight...