Car dealerships (and other businesses) do not operate like a manpower company - i.e. they are not hiring-out their mechanics at an hourly rate. .
In fact, the mechanics' salary is probably only a marginal factor in their operating costs.
Firstly, for every mechanic whose time is chargeable, there are an x number of people who get paid their wages but whose time is not charged to the customer by the hour - the customers do not pay an hourly rate for the time of the service advisor who deals with them, or the time of the service manager who sits in his office, or the time of the bookkeeper processing the invoices and payments in the backoffice, or the receptionist at the entrance, etc. etc. And this is before we even get to the cleaning and security staff.
Then, there's 15% employer's NIC, 3% workplace pension, and staff training and staff holidays (which is when staff get paid but no customer is charged).
And last, there's rent, business rates, electricity, heating, IT.....
And after all this.... the business needs to turn a profit so that the owners get their ROI.
Not making excuses for overcharging customers, just pointing out that the mechanics' wages is far from being the main factor in the hourly rate charged to the customer.