Tricky one that really.
Without divulging too much detail, I would certainly have explained that running a unit/van/updating H & S legislation/having a quality insurance policy to protect him and his property in case of mishap/paying tax & NI/water costs/replacing expensive poles & brushes/going down the pub or bookies/having six holidays a year etc etc can run quite expensive and a decent hourly rate is needed in order to provide a decent, bona fide business. (OK, delete the pub/bookies/holidays then

). I do believe that if someone is running their business with the admin all properly done and paying for premises, an "on the glass" rate approaching £1 a minute can be justified. Not that I achieve that - though it is possible on some jobs.
Maybe the best way is to do all the jobs before collecting the money - thus making it harder to put a stopwatch on you. Also, any trad bits (front door maybe?) get left to the end so that it is even harder to time things.