You don't have to be a member of a scheme or incur extortionate Building Control fees to sign off for part P.
See here
"The BCB can also use the IET/BCA alliance document. The Institution of Engineering and Technology (IET) and the Building Control Alliance (BCA), co-produced this document and it is downloadable free from the IET website, to gauge an individual’s level of electrical and Building Regulations competency. The BCB can then use this information to adjust building control charges accordingly, based around the level of risk an individual poses and how much work the BCB would incur to confirm this. Using this method, a BCB may accept a BS 7671 Electrical Installation Certificate (EIC) from an un-registered competent person."
And here
"I ET-BCA Guidance for Local Building Control Bodies: Electrical Installation Qualifications and Competence (within England and Wales)
The purpose of this new peer reviewed Guidance document is to outline the skills and competences required for an individual to carry out electrical installation work within dwellings in England and Wales. Specifically work that falls under the remit of Part P of the Building Regulations and referred to in Approved Document P.
An individual’s competence level or skills are afforded a risk rating, explanation and action outcome within the Guidance document. These may then be used to calculate if any additional work may need to be taken into account by a local Building Control Body (BCB)."
If you have the skills and the qualifications then there's no reason for a BC officer to visit, so the costs should be no more than the admin fee to issue the certificate. I would imagine £15 to £20.
If your only doing a few Part P jobs a year then this route has got to be cost -effective..
All the schemes do is take some of the admin, ie. The requirement to keep a register of competent persons, out of building controls hands.
When Part P was enacted there was no intention to give the scheme providers an absolute monopoly, But that's how it seems to have become, especially with the schemers perpetuating the myth that that is the only way to go.
I don't know how easy this route is to achieve but it can't be any more onerous than joining a scheme. I don't know anyone who has done it. Perhaps there are some on here who have? It doesn't seem to be a well know alternative.
I should add that this route appears to be just another scheme and there is nothing to stop you approaching BC personally with your certificates etc. And they may accept that you are a competent Person.
My quotes are from here if anyone wants to read more.
Building Regulations - Part P certification schemes - IET Electrical - http://electrical.------.org/building-regulations/part-p/certification-schemes.cfm