The more I think about the 3rd party signing off work the less I worry about it. Round my way by far the worst electrics I see are ones done on extensions and thus are "covered" by LABC planning permissions rather than Part P sparkies.
If a non registered sparky does some work and gets his registered "mate" to sign it off the unregistered bod won't earn as much and the guy signing off will want paying, so there isn't likely to be much of a saving for the client.
In this senario, customer has issue, builder gets sparky back who says there isn't an issue, customer who has name of tester calls them back and they say the issue is down to the installer - neither want to help.
So who wins? Sounds like the situation we have now will continue.