Do I need to hire a certified electrician/plumber to install water and electricity supply for a kitchen to ensure any following damage is insured by the building/home insurance or can I do it myself without a formal qualification?
Whatever you install must be checked by a person with the qualification to do so legally. If this doesn't happen, then you are in legal trouble. For example, an electrician three days before finishing his apprenticeship should know everything how to install the electricity supply for a kitchen safely, but needs someone else to check for it or sign for it. It might cause you trouble for example when you try to sell your home and cannot prove that the kitchen was installed safely.
If you don't have the capability of doing the job safely, and risk your life and the life of others, that's of course even more trouble. You'd be risking the life of your partner, children, visitors etc.
If you did actually cause damage through your unqualified work, I can't tell you what will happen in court. I can tell you that the insurance will do whatever is in their power to avoid paying, and that's not something you want to do. Find a qualified electrician to check your work if it's done. If your work is no good, it will fail. If it cannot be checked (plumbing covered up in brickwork), it will fail. In either case, you need to fix it somehow.
Faulty plumbing can be very, very expensive. Faulty wiring can kill people. Faulty gas connections will kill people.