Substitution
One of the three core IR35 tests — a genuine right to send a qualified substitute in your place indicates outside IR35.
Substitution is one of the three primary employment status tests used in IR35 determinations (alongside control and mutuality of obligation). It asks: does the contractor have a genuine, unconditional right to send a substitute to do the work?
A true contractor can send an equally qualified person in their place without needing the client's permission (or with only a reasonable right of approval for qualifications). An employee must personally provide the service.
For substitution to be effective in an IR35 argument, the right must be real — not theoretical. Courts have found that if the client has never actually accepted a substitute, or if contracts contain a substitution clause that would realistically never be exercised, the clause carries little weight.
Umbrella workers are employees and substitution is not relevant to them — they provide services personally as direct employees.