It's a tricky one, because the vast majority of contractors won't exercise it unless they have to. In my own case if I have a few months off then a 6 month contract, I probably won't have any time off in that whole 6 months.
It's never been tested in court (that I know of) but I'm pretty sure substitution clause being invalid unless used wouldn't hold up. Wish I could find the case I was thinking of that was recent, but I did find on searching my emails that Kaye Adams had won her case and had not only never used a sub but thought she couldn't, even though the clause was in the contract.
Source:
https://www.contractoruk.com/news/0014950kaye_adams_ruling_substitution_will_be_comfort_contractors_lawyer.html?utm_source=NL&utm_medium=News&utm_campaign=IR35
Biting the hand that feeds IT
