Qualified Supervisor is a job role/description, not an actual qualification. A self-employed registered electrician has exactly the same qualification requirements as does an employed one whose job is to be the QS for his employer.
The way that electrical installation businesses of any size usually work is that they have qualified supervisors who supervise and direct the work of unqualified/inexperienced workers - traditionally I guess these would have been apprentices and improvers. Provided the supervisor is actively involved there's nothing really wrong with this, and he can effectively look after the work done by several people. With Building Regulations certification it is the firm that is registered, and the firm that certifies the work, because it is all done by the firm.
In this case though it sounds like he's not been involved, and worryingly there was never any plan for him to be involved, hence the extra £200.
And if the electrician does not employ the kitchen fitter, or if they aren't both employees of the same firm, then it really is stretching things for him to claim the role of Qualified Supervisor.