It's not actually illegal to do your own electrical work, though you'll need to get some types of work inspected. HOWEVER, your insurance company will absolutely seize on it as an excuse not to pay if your house burns down (most insurance policies will require that anything but the most basic work is done by qualified people).

At least it’s not as bad as in the US. As I understand it you only need to get it inspected and signed off by a licensed electrician if you DIY, no need for permits from the local council.

You can if you're a qualified electrician. Or if not do it yourself and get a qualified electrician to approve it afterwards.

Depends. You can certainly replace broken or worn socket/switch faceplates.