One reason to stay away from non-trivial plumbing and electrical work and to be careful about any work that is in a designated high risk area or otherwise covered by building regs (e.g., work around a fireplace/chimney, kitchen, bathroom) is that you will struggle to claim on insurance if something goes very wrong, either when the work is being carried out or afterwards. Strictly speaking, even when you are paying a trade for compliant work that is covered by building regs, you need to inform your home insurance provider several weeks in advance. Very few people do, of course. Smaller jobs that are notifiable are simply not financially viable as a DIYer because you will be paying the council hundreds of pounds to check your work when a competent person can sign it off themselves. In other words, you should know what you're getting into.