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Thanks very much. I thought the garage wiring would end up being a professional job!
I think a good place to start is an EICR certificate and then ask for opinions on the scope and cost of garage work.
fizzy.bleach-
It's not a bad approach but a lot of electricians will not want to be drawn into specifying a job for free. You can always ask for a price and what's involved, it won't help you to avoid the fact that it needs signing off but at least you'll get an idea of cost.
I've not done any but my understanding is that most modern garage installations have an armoured cable which can be buried or surface mounted without further mechanical protection. Clearly you need one with a high capacity (read expensive). You need to protect that cable from an overload with a breaker, perhaps they would split the incoming supply after the fuse and put a large single breaker on it. Then you put a consumer unit in the garage which has RCD protection. RCD protection is a requirement for any sockets near a door or window that could be used to supply power outside (driveway/garden etc.) Stands to reason a garage is going to have RCD protection on the sockets. You might have a loop if you want 32amp capacity or just a selection of sockets on a star or radial layout. You won't really need 32amp in most cases. Lighting on a 6amp circuit.
So basically a lot of money and effort to achieve what you already have which is power in the garage. You can get RCD protection on the breaker in the house or just use an RCD plug socket with anything you are using in the garden. If you have a lot of powerful tools in the garage and find you are tripping the breaker, then it might be worth upgrading but otherwise you could spend the money elsewhere.
If you get an EICR they will likely flag things which would need to be done to meet current regulations but the regs are not applied retrospectively so until you start some serious work you don't need to address them.
Airhead
There are a lot of things you can do in the house without having to inform building control which is Part P. Understanding Part P and it's scope is a fair sign that you have some knowledge of electrical safety and the modern health and safety regime.
With the garage job it seems you don't really know where to start in terms of how you would change the supply to meet modern demands so you're going to need professional help or take a fairly long course in electrical engineering to equip you with the answers. It's not impossible to figure out what needs doing but you would still need to have it signed off. Some electricians are members of schemes that don't allow them to sign off third party work, the regulations also require that they are involved in the job from the beginning, when that gets complicated it's easier just to do the job yourself if you're a professional electrician.
One of the easy rules of Part P to bear in mind is 'New Circuits need signing off'.
Adding sockets is fine as long as you don't add spurs to spurs. Moving sockets is fine as long as you follow the simple rules for allowed routing. You're supposed to fill out minor work certificates but I find it hard to type that without smirking :)
I started to get involved in domestic electrics 20 years ago because I found it fascinating, there's a lot of information out there if you want to learn but it's a pretty big subject. I'm qualified to take on smaller problem solving jobs and it fits with my joinery and renovation work.