Best answer by Transparent
These two devices are the property of the Distributed Network Operator (DNO). If I correctly deduce from your online handle that you're based in South Wales, then your DNO is Western Power. In my experience they are an excellent company and really try to help customers.
The usual procedure for relocating the Distribution Board (proper name for the "fusebox") is that your qualified electrician arrives on-site and phones to notify Western Power that he will be removing the main 100A fuse. He then agrees with them a time later that day when they will attend on site to check the integrity of the new feed cables he's installed to reach your chosen position for the Dis-Board.
Your electrician is then allowed to cut the DNO's seal and pull out their fuse.
Once they arrive, it is a matter of a few minutes for the WP Engineer to check all is OK and reseal the fuse. I believe there is no charge for this, because that policy encourages people to do things correctly and communicate with them.
It is not necessary for your new Dis-Board to be fully functional in order for the DNO to re-seal their fuse. The basic minimum is that the correct spec of cables is connected to a 100A 2-pole switch disconnector (the big red switch!) in the Dis-board. Your electrician is the one responsible for everything that goes into the Dis-board itself and the wiring beyond.
Once he's completed building the Board with the required MCBs and RCDs he will issue you with a copy of his Part-P test sheet (that name is taken from Part-P of the Building Regulations). Your actual Part-P certificate usually arrives a couple of weeks later from the certifying authority with whom the electrician is registered. Keep it safe.
You get no certificate from the DNO and you have no direct contract with them. Their role is to serve the needs of your electrician.
If you are undertaking extensive refurbishment of the property it is sometimes the case that you can't finish the installation of the new Distribution Board for some time, possibly because the wall where it is to be fitted needs replacing.
I faced such a situation because I needed to re-route all the cables beneath a floor to which I didn't then have access.
My solution was to first get my friendly local electrician to install a tiny Dis-Board, just big enough to house a 100A switch disconnector, separate to the ones in the three Dis-boards I was going to replace.
The DNO engineer only had to check the cables between the meter box and this separate Main Switch. Whenever my electrician and I needed to do further work on my circuits, we could kill power to the whole house without having to cut the metal seal on the main fuse and recall Western Power back to site.
Hope that helps.