Bob,
The old Form 2 was included for commercial reasons where by large works could then legitimately be designed by different companies to the installer and the inspection & testing company.
The last bit is of heavy debate in the industry as it is impossible to correctly I&T a completed installation as is often done, the installation MUST be inspected during its construction to ensure full compliance with BS7671.
It also serves the purposes of large organisations whereby they may have a design team, an installation team and an I&T team. Again the latter part being of debate for the same reasons.
Remember however, it is the "person responsible", i.e. take a large complex design, you may have a lead engineer with several subordinate engineers designing different sections of the installation, it would be head of the engineering design team who would sign the certificate.
I say the "old" Form 2 because with the last revision this designation was removed.
To turn your argument back on itself, if you refer to BS7671:2008 Amd1 2011, you will see on page 389, that it is allowed for a single person to undertake all roles?
So there is no "need" or requirement under BS7671 to have different skill sets, it is recognised that a single person can be competent to undertake al roles, this person would be called an Electrician.
So, BS7671 allows for the designer, constructor & I&T to be different entities, however, there is absolutely NO requirement that they are, different entities or persons.
The advanced courses, the "old" 2391 & 2400 (very old) are there for competent electricians to broaden and enhance their skill set. Take them as CPD if you like, however, a qualified electrician must be able to undertake all of the requirements of BS7671.
We are not talking about designing, installing & I&T'ing a new Steelworks here, this is 2 off 400V (415 is no more by the way!), 3 phase socket outlets, which sound as if they are within sight of the DB.
This is a task that should be completed by a competent electrician with no problems what so ever, including any design calculations, the installation & the I&T.