Gas, Electricity, Telecommunications and Water suppliers are governed by independent regulators you can take complaints to. Advice centres can tell you about these, or they are generally listed in the telephone directory, or on the back of your bill. Before you enter into an agreement Do extensive research before entering into an agreement for expensive work, and get several detailed estimates or quotations. An estimate is usually taken to be a rough guess and a quotation a fixed price for the job. Consider an insurance backed guarantee for expensive work but investigate it thoroughly first.
Some trade associations have such schemes.
There are very few trades or businesses where the law requires qualifications. But any claims made must be true, and you should check claims of membership of trade associations or approval by official bodies. Extended guarantees for domestic equipment and break-down warranties for cars should be researched carefully. They may not be suitable for your purpose or good value for money. Long guarantees are only as good as the company that issues them.