Re: Yes, blame the tool
The substitution clause is absurd in IT. Only in some major project with hundreds of cattle programmers would it be reasonable to substitute someone chosen by the original contractor rather than the client.
If I contract with a consultant, it's because I want to purchase their specific experience for a short period. It isn't employment in a form relevant to employee benefits, tax etc. I don't expect to pay those any more than I expect to pay a medical consultant's NI.
HMRC are engaging in the same sort of practice as the people defining what a planet is : they start with the answer they want (that Pluto is, or isn't) and then try to tweak the rules until they get that outcome. It works for individual cases but will never produce generic rules.