The company my local LABC use for inspection and test clearly state on their website that they can do 3rd party certification. I've also heard of several other NAPIT electricians that offer this service and are accepted by the local LABC's.
But those are not, and cannot possibly be, certifications of compliance with the Building Regulations, so the work still has to be notified to LABC in advance, and the fee still has to be paid to them.
Are you sure that you are still not a bit muddled on the difference between, and who can issue, certificates showing compliance with the Wiring Regulations, and certificates showing compliance with the Building Regulations?
If LABCs are happy to accept a certificate of I&T results from an electrician, and do without any checks that the work also complies with all the other parts of the Building Regulations then shame on them, but I guess they feel they are allowed to neglect their responsibilities in that way.
How the economics of DIY + LABC notification fee + electrician's fee stack up against engaging the services of a registered electrician I don't know..
You get a full EIC with a note outlining that the install was not undertaken by themselves.
So who signs the part for design compliance? Can't be them, as they didn't do it.
Who signs the part for installation compliance? Can't be them, as they didn't do it.
And given that fails, LIMs N/As etc are not acceptable on the test schedules for the I&T part of the EIC, unless they inspect at 1st & 2nd fix then they can't sign that part either...
I've not heard of any complaints so far from the LABC's around here.
You wouldn't - they're raking in fees and providing SFA in return apart from a dodgy piece of paper.