I see no reason a qualified person cannot do their own work/inspections but as I mentioned before - I'm not electrically qualified - however I sub to the guy who did my inspections and I've done a lot of that sort of work on his behalf.As you say, nothing to stop it. I think the OP is thinking that if anything were to happen, then someone might suggest he didn't do the job properly because he had a personal vested interest in passing it. Personally I'm happy to take that risk, and should anything happen then I'd be "let me refer to my notes, otherwise known as the EICR certificate - yes I tested that X, and at the time it tested as <whatever figures>". And here's my daily calibration log for the MFT showing that it hadn't drifted either before or after being used for these tests.
As Ericmark says, I think most landlords do "do the right thing". Unfortunately, as long as there are those who don't and agents that support them, then we'll be stuck with all the legislation. It will be interesting to see when No2 daughter's landlord decides to get one done, and even more interesting to see how many things it fails on - if it doesn't fail, then someone will be getting thrown out of their scam membership.
I don't fancy the courtcase [god forbid it should ever happen] and having to explain how I [as a non qualified person] had done the CU change and inspection and someone else has signed the documents.