My local small independent garage charges £40 per hour plus VAT plus parts.
Now their rate is the lowest in the area and they do only that which is necessary, contact you for questionable items (e.g pads will 100% be required to be changed at next service) and then let you decide if you want them to do the work this service or next. They are also 110% cautious so you will never be returned a car that in their opinion is dangerous or unsafe.
For them basic oil change, filter changes (oil, air, pollen, fuel) and basic serviceability and safety check is normally billed at 1 hour. If pads, discs, etc need changing then they charge at the industry Auto Data set time for the task. If they take longer they absorb the cost. If they are a little bit quicker then the standard time rate still applies. Now bearing in mind that Auto Data times are very difficult to achieve if you attend to quality and care then it is highly unlikely an honest garage will equal or better the Auto Data rates.
Most main dealers charge a minimum of £80+ VAT per hour. BMW and Mercedes are probably in the region of £120 per hour. (not checked recently).
To change a set of rear pads is IMHO a 30 minute task (MINIMUM) from start job start (get in car, drive to lift, etc) to job end (park car back in customer bay). So on this basis £20+VAT + parts is a very cheap and fair price to pay based on £40 per hour.
A cautionary point! If you are actually charging this person/persons to do this servicing work the I STRONGLY suggest you get proper liability insurance. You should be able to obtain this (if your qualifications are valid & accepted) for a modest fee (probably £100 to £200 for 3 million pounds of cover but I have never formally enquired for motor trade operation).
