Sorry the 5:2 book addresses this issue and says you can’t eat your exercise calories on fast days.On page 122 of the Fast Diet it says” do not however increase your fast day food allowance to compensate for calories burnt through exercise, on a fast day stick to 500 or 600 calories.”
So it does encourage you to exercise if you feel like it. I for instance usually swim 60 laps but on fast days I might only swim 40.

Remember that this diet wasn’t intended for weight-loss to begin with, so you’re coming at it from the wrong perspective altogether. The goal is the IGF1 effect, which comes from how much is going in (triggering a “famine” response in the body), not from the net difference between going in and expending. If your sole goal is weight-loss, then perhaps you can look at it from this point-of-view, but don’t be surprised if you don’t get the health-benefits that this diet was actually designed for to begin with. I also recall part of the documentary in which it was pointed out how unlikely it is to exercise off what you have eaten (Michael had a coffee and a donut, and it was pointed out hours of exercise would be needed to work that off. Don’t remember the exact amount, but the pointlessness of that idea was pointed out).