That's utter nonsense redvanman there is no requirement for work to be signed off by a member if such a scheme if you are not registered. Show me where in the regs it states this.
I don't know where you heard this but you have been misinformed
So if you have wired an extension and the builder wants it signed off then what happens.
Part P is a legal requirement and it clearly states on the first page that if you are not a registered person then you can use a registered third party or the building control body who intern use a registered third party