Re: Substitution?
I would say it would be very difficult for a developer to be self employed under IR35. What I would do in this position is form a small development company with 2-3 other developers and then bid of packages of work. Any client would then be asked for either a shared login or an individual one for each developer. Who does the actual work internally is then of no consequence to the client.
In the event of a client wanting someone in their offices during their working day I would get all 3-4 of us cleared and vetted so that we would actually be in a position to make a substitution.