Client willing to accept a substitute is not a get out of jail card.
There are many rules around that. For instance - if it was possible for the client to hire the substitute directly (which is almost always possible - a matter of money) - then the substitution does not count.
It will also likely not count if you have not already used the substitute with that client. The substitute also must have not worked with the client before.
Then HMRC looks whether use of substitute is practical - for instance if the replacement would have to spend considerable amount of time to get up to speed.
Substitute also won't count if for instance you write the code and then send someone to present it. Substitute must be doing exactly the same job as yourself.
Ultimately the HMRC can always say that willingness to accept a substitute is to make the contract out of scope.
The system is basically set up to ensure the only "right" way is to have determination in scope.
Biting the hand that feeds IT
