robo123,
Alas the nature of the work you plan to carry out, will be notifiable. That does not mean that you can not DIY it. You just need to make an application to building controls under electrical work, prior to starting work, explaining what you are doing and how you intend to perform it. They will charge you a fee, that sometimes for the work being done is astronomical, then come out and inspect your work. They may or may not depending on the way your council is run, ask you to supply schedules of inspection and testing along with an EIC. or they may contact someone to do that for.
The installation you wish to make, if you do have some fundamental knowledge of electrical installations, inspection and testing, may not be beyond you.
But by some of the question you have asked, I don't quite think your up to full speed just yet.
If you can clarify some of the questions asked to you in earlier posts, regarding the supply side. Earthing arrangement, cable supply type of house to garage the size, route taken and method installed, also main fuse rating and the current demand already applied to it. If RCD protection is installed and size of earthing conductor and main bonding to gas/electric/oil/metal work etc.. Some of us kind people on here, may be able to offer some constructive help.