You can install electrics in your garden (many people do it for lighting, a pump for a pond, sockets for lawn mower and so on).
But it has to be done properly or could kill the people using it if done badly.
I saw this on one web site
Most domestic electrical installations fall under Part P of UK building regulations. In order to be able to prove you have a safe, compliant electrical system in your shed, you’ll need to do one of two things:
•Employ an electrician who is Part P compliant, or
•Notify your local authority who will then inspect it (at a cost of several hundred pounds).
Employing a Part P compliant electrician is easier and more affordable.
If you ever sell your home, the new owners will understandably want to know the job was done properly, so don’t ignore this legal requirement.