You can do you own wiring if your considered to be competent but you cant do it for anyone else. Although the exact explanation of competent is a bit vague.
As for your workshop wiring this can require a bit of "expert knowledge" shall we say. Is the workshop part of the house or attached to it or is it seperate from it? Do you know what type of earthing arrangement your house supply has? Most of the time the recommendation is for your workshop to have its own earth, ie make the workshop a TT type arrangement which is the earth rod in the ground.