What prevents a tradesman from billing a client an unreasonable fee after the work is done? For example, a plumber may quote provisionally £100 verbally (or not even give a cost estimate as is common here in the UK), do the work, then invoice £10,000 for a minor job that took maybe an hour. What consumer protections are in place for a case like this?
You reply "sue me". Then either nothing happens, or he takes you to court. In court, you state that you had a verbal contract for £100 pound, then you get an expert witness who states that this repair is not worth more than £100, and £10,000 would be a completely unreasonable amount. To make him squirm, you ask the tradesman for tax statements that prove he has charged people these amounts and what profits he made - shall we bet that he isn't paying taxes properly?
The judge then has to decide what is more likely - that you entered a verbal contract charging £100 for a £100 job, or that you entered a verbal contract charging £10,000 for a £100 job. His chances of getting a judgement against you are basically zero.