Things could be clearer in the electrical world.
The what is allowed without notification is laid out in building regulations part P. In Scotland a Warrant, and Wales and England is not the same.
For notifiable work, the LABC inspector can decide what he allows you to do. The main problem is the inspection and testing, which can be charged for on top of the LABC fee. So it can work out more expensive to DIY than to use a scheme member. The second is the time, they are allowed 2 weeks to inspect, so if they say your not to power up until inspected, then one can be without power for some time.
But technically there is nothing which can't be done DIY. As to gas, as far as I am aware, the gas safe restrictions are only for work which is charged for, DIY work is not stopped, I think it should be stopped, but the restriction just stops untrained tradesmen.
There are some electrician jobs which in order to be connected to the grid, the person doing them does require extra training, EV points, solar panels, batteries etc. Without the training and scheme membership it is hard, but not impossible, to get them connected to the grid.
For the electrician he should always inspect and test, there are two forms, the minor works and the installation certificate both free downloads from the IET. Anyone doing electrical work is an electrician. We all know often the DIY omits doing inspection and testing, but there is nothing to allow this omission, but without the test gear it is near impossible. We are looking at £500 to £1000 in general for the test equipment, it can be hired, at around £70 for the week.
It is not plain as to if you can omit some tests, it says enquiry is a valid method, so looking at an existing installation certificate and copying some details to the new one is it seems permitted, as is having an EICR done and again copying across the results.
I have looked at the installation certificates issued for work on my own house, in the main solar panel installation, and the certificate was down loaded, so there is no signature as such, the certificate has to be also sent to the LABC who publish the compliance or completion certificate, I assume they get the signed copy?
But to be frank, the details are rather vague, and I can't see how anyone could say today who did what by reading the certificate, I know I doubled up on my battery size, and I forgot to download the updated paperwork.
To my mind the paperwork should be posted to one, this having to down load it seems wrong.